Fundamentals and characterizations of scratch resistance on automotive clearcoats

Abstract

As original equipment manufacturers (OEMs) strive to deliver improved coating performance with a sustainable footprint, opportunities for innovation are emerging, particularly on improving mechanical properties, appearance, and solids content. Resistance to scratch and mar damage is one of the key performance attributes that has been emphasized by both OEMs and consumers to maintain a vehicle’s appearance and corrosion resistance over its service lifetime. Fundamental methodologies including instrumented scratch measurements at multiple size scales are used in this work as part of a product development strategy to better understand the scratch and mar behavior of automotive topcoats. This study compares physical properties of several melamin-formaldehyde and isocyanate cured clearcoats over the appropriate basecoats. Micro- and nano-scratch techniques were employed in combination with industry standard method, Amtec-Kistler carwash to identify performance differences under different scratch conditions. Mechanical and viscoelastic properties of the coatings were studied using tensile tests and dynamic mechanical thermal analysis (DMTA) to better understand the failure mechanisms associated with plastic deformation and fracture at different scratch scales. The information gathered from the above testing protocols is used to analyze coating performance in terms of the contact strain, transitions between elastic – plastic behavior, coefficient of friction and stress localization.

1. Introduction

A typical automotive coating system consists of four layers divided into two main categories: protective and aesthetic [1], (see Fig. 1). The protective layer is usually composed of a phosphate pretreatment layer and an electrical deposition coating (E-coat) applied directly onto the body-in-white with a thickness range of 17 μm–22 μm. The main purpose of these protective layers is corrosion prevention. The first aesthetic layer is usually a 20 μm–30 μm primer layer for stone chip resistance, smoothing, and some ultraviolet (UV) resistance. The aesthetic topcoat system consisting of two layers - basecoat (BC) and clearcoat (CC) - was introduced to the automotive market in the mid-1980s with the intent to improve the initial and long-term appearance and durability [2]. The basecoat brings color and visual effects with a 10 μm–20 μm film thickness, while the clearcoat is the uppermost layer. The clearcoat is engineered to provide a glossy appearance and to protect against a wide range of environmental stresses as well as mechanical damage from stone chips, scratches, marring, etc. [3]. The clearcoat film usually has a film thickness range between 30 μm–50 μm [4]. All the layers combined are just a tenth of a millimeter thick, making automotive coatings highly engineered films no thicker than a human hair.

Fig. 1.

Simulated load profiles [7] and scheme of scratch depths caused by different activities.

Despite the high quality of current automotive coatings, there is still a huge demand for coating performance improvement and test correlation methodology, particularly for scratch resistance [5]. According to JD Power 2017 U.S. Initial Quality Study^SM, more than 50% of consumer complaints are associated scratch/chip imperfections. Therefore, scratch resistance of automotive clearcoats is of great interest to car manufacturers since improvement here can help maintain long-term appearance of vehicles, reduce warrantee claims and increase vehicle resale values [5]. Consumers also benefit from improved scratch resistance of automotive coatings since the average length of ownership for a new vehicle continues to increase. According to the 2015 IHS Markit Study, buyers held onto a new vehicle for 6.5 years on average. This length of ownership represents a more than 50% extension as compared to 2006, when new vehicles were owned for an average 4.3 years [6]. Therefore, developing a clearcoat with permanently high scratch resistance has become one of the highest research priorities for automakers and paint manufacturers.

When scratches are a few microns in depth and width on clearcoats without fracture, they are referred to as mar [7]. This type of shallow defect often results from handling during assembly, carwashing, or blowing sand and dust. A single mar is difficult to visually detect, but the damage becomes more obvious when many mar defects are in close proximity (e.g. carwash) [7]. In general, the large, easily visible scratches caused by keys, fingernails, tree branches or shopping carts are classified as fracture scratches [2]. They appear white due to fractures along the scratch groove and range in depth from under 25 μm to over 1 mm due to much higher scratch forces, which means they sometimes damage the paint finish down to the bare metal substrate resulting in corrosion issues [7] (Fig. 1).

Field coating failure mechanisms are complex phenomena and are difficult to accurately reproduce in the lab. The duration of field tests is typically required in months or years to complete, thus the drive to develop a reliable and fast laboratory test to simulate scratching and to quantify scratch resistance on an accelerated timeframe [5]. Quantification of scratch performance can be complicated due to unknown variations on both damage processes and how the test was performed by researchers [8]. Scratch resistance involves a complex interaction between viscoelastic, plastic flow and fracture [9]. Currently, the automotive coating industry and original equipment manufacturers (OEMs) use relatively simple test methods to evaluate clearcoat scratch resistance and try to predict its field performance [10]. The two most popular industrial test methods are the Crockmeter and Amtec-Kistler Carwash. The first one is a dry method that simulates the most common scratching scenarios, especially in-plant handling and contact with abrasive surfaces in use. The second one is a wet method simulating carwash damage. The coating performance is based on visual assessment and gloss loss measurements. However, results from both assessments can drastically differ and are often contradictory between laboratories and field feedback, even for relative ranking [8]. Therefore, to develop new, improved scratch resistance coatings and meet customer demand, it is crucial to develop a test method that will help better understand the mechanisms of multiple types of scratch processes, as well as establish correlations between different tests and field results.

A number of instrumented scratch and indentation instruments were developed in previous decades to probe the mechanical properties of coating materials [8–11]. A single point scratch test is a preferred choice [8–11]. In order to simulate such small scratches and get a quantitative value for viscoelastic recovery at this scale, instrumentation with 1 μm to 2 μm size tips, sub-nanometer depth resolution and nano-newton load resolution is required [9]. Severe scratches require a larger scale tip in a range of 50 μm–200 μm with several newton loads. However, more fundamental studies need to be completed to (i) better understand scaling behaviors of multiple types of scratches, (ii) improve methodology and metrology to correlate viscoelastic properties (e.g., fracture energy, glass-transition) to scratch resistance, and (iii) establish correlations between instrumented scratch testing results and overall field performance.

From chemistry or formulation standpoints, common approaches used by coating suppliers to improve scratch and mar resistance of automotive clear coats are increasing crosslink density within the polymer network, addition of hard nanoparticles, or self-healing technologies [12]. These methods have been shown to enhance scratch resistance or scratch repair. However, these changes can negatively impact other properties such as toughness. Further innovation on resin and cross-linking chemistry may be needed to enhance clearcoat scratch performance.

In this study, fundamental methods including nano-indentation, as well as nano-, micro- and macro-scratch measurements, tensile and DMTA measurements were applied to several clearcoats cross-linked with melamine formaldehyde and isocyanate (see Fig. S1) for automotive applications to better understand scratch methods and metrology. These techniques were employed in combination with industry standard Amect-Kistler carwash to help better understand the scratch and mar behavior of automotive clearcoats by quantifying the scratch failure in plastic deformation and fracture energy to correlate to field performance. This approach also helps to understand how molecular network parameters influence clearcoat scratch performance. The current study also shows that both basecoat and clearcoat can affect resistance to scratches at different intensities and across length scales.

2. Experimental Procedures

2.1. Materials and sample preparation

Automotive OEM coating systems in this study were chosen and grouped by matching commercial basecoats (BC) for the purpose of representing a variety of clearcoat formulations as shown in Table 1. All clearcoats (CC) were selected to ensure good compatibility with the specific basecoat systems in which they were paired. The criteria of good compatibility are that no visible wrinkling or shrinkage appears on the clearcoat surface and the gloss was greater than 85 gloss unit.

Table 1.

Details of automotive coating systems utilized in this study.

Basecoat name	Clearcoat name	Sample name	System layering	BC Color	Cross-linking chemistry
1	A X	A1 X1	CC/LS-SBBC/primer/e-coat	Silver	1 K polyol-melamine formaldehyde
2	B X	B2 X2	CC/HS-SBBC/primer/e-coat	Silver
3	C X	C3 X3	CC/WBBC/primer/e-coat	Black
4	D Y Z	D4 Y4 Z4	CC/WBBC/primer/e-coat	Black	2 K polyol-isocyanate/PU

As shown in Table 1, a sample name consists of clearcoat name and basecoat name, in which a letter represents a clearcoat and a number represents a basecoat. For example, coating sample A1 represents that the sample was prepared using clearcoat A over basecoat 1. For a freestanding film sample, only a letter is exhibited as it was prepared from a clearcoat formulation without basecoat, such as clearcoat A, B, C etc. discussed in Section 3.1.

For basecoat system, basecoat 1 is a low solids solvent-borne base-coat (LS-SBBC); basecoat 2 is a high solids solvent-borne basecoat (HS-SBBC); both basecoat 3 and basecoat 4 are water-borne basecoats (WBBCs). For clearcoat system, all clearcoats A, B, C and D were commercial clearcoat formulations and applied by Eastman Chemical Coating Development Laboratories, while clearcoat X, Y and Z were formulated and applied in Eastman Chemical Coating Development Laboratories. Therein, clearcoats A, B, C and X are based on 1 K polyol-melamine formaldehyde while clearcoats D, Y and Z are based on 2 K polyol-isocyanate polyurethane (PU) cross-linking chemistry.

All commercial basecoats were sprayed onto 4 inch × 12 inch (10.16 cm × 30.48 cm) pre-primed steel panels (ACT, Hillsdale, MI) using a conventional spray method at a dry-film -thickness (DFT) of 15 μm to 20 μm. All clearcoat formulations were sprayed onto the basecoats at 45 μm ± 3 μm dry-film-thickness after a 3 min room temperature flash-off for solvent-borne basecoats or 5 min 80 °C dehydration for water-borne basecoats to simulate the industrial standard compact paint process. Following a 15 min final flash-off, panels were baked at 140 °C for 30 min. To ensure all the samples have been properly cured, Solvent Resistance Rub Test (ASTM D4752) using Methyl Ethyl Ketone (Eastman Chemical, Kingsport, TN) were performed for baked coating and all the samples passed 300 rubs.

Free-standing films with 45 ± 5 μm dry-film-thickness (DFT) were prepared from liquid clearcoat formulations for dynamic mechanical thermal analysis (DMTA) and uniaxial tensile testing. A uniform clearcoat layer was cast onto Teflon coated aluminum (DW 350GR, DeWaL Industries) by using a wire-wound drawdown rod (Paul N. Gardco Company, Pompano Beach, FL) with a steady drawdown speed around 15 cm/s. The formulation viscosity was adjusted to 28 s at DIN-Cup #4 prior to drawdown. Films were baked at 140 °C for 30 min and gently peeled off the Teflon coated aluminum. 9 readings were taken from each 20 cm × 20 cm free-standing film. The average DFT with a standard deviation for each film was given in Table 3.

Table 3.

A comparison of mechanical properties measured from tensile tests for clearcoats. The uncertainties are standard deviation from five replicates. The original stress-strain curves are shown in Fig. S2a–b.

Sample name	DFT (μm)	Strain-at-break (%)	Strain-at-yield (%)	Ultimate tensile stress (MPa)	Young’s modulus (GPa)	Toughness (MPa)	E/σy
A	45 ± 4	3.3 ± 0.0	–	42 ± 4	1.84 ± 0.18	0.9 ± 0.2	–
B	44 ± 3	2.2 ± 0.3	–	50 ± 10	3.10 ± 0.30	0.8 ± 0.1	–
C	45 ± 5	1.8 ± 0.4	–	30 ± 3	2.07 ± 0.16	0.4 ± 0.1	–
X	46 ± 2	8.0 ± 3.0	4.4 ± 0.3	44 ± 2	2.04 ± 0.18	2.9 ± 1.1	49 ± 5
D	43 ± 3	7.7 ± 1.3	4.1 ± 0.5	50 ± 3	1.92 ± 0.16	3.0 ± 0.6	39 ± 4
Y	46 ± 4	7.8 ± 1.7	4.9 ± 0.2	66 ± 3	2.30 ± 0.14	4.1 ± 1.2	35 ± 3
Z	44 ± 4	8.0 ± 2.0	4.4 ± 0.2	56 ± 3	1.85 ± 0.14	3.3 ± 1.1	33 ± 3

2.2. Instrumented scratch and indentation tests

All the instrumented scratch tests used a patented three-stage method: pre-scan (C₁), scratch (C₂) and post-scan (C₃) [13], as shown in Fig. 2. During the pre-scan and post-scan, the tip was used to profile the surface under small load before and after scratching. The scratch was performed under progressive load. These two profiles were used to correct further penetration signals yielding a true penetration depth (Pd) and residual depth (Rd) as shown in Fig. 2. Pd and Rd can be calculated by Eqs. (1) and (2):

$$\mathit{Pd}=C_1-C_2$$ (1)

$$\mathit{Rd}=C_1-C_3$$ (2)

Scratch recovery can be expressed by Eq. (3):

$$\mathit{Scratch\; recovery}=\left(1-\frac{\mathit{Rd}}{\mathit{Pd}}\right)\times 100\%$$ (3)

Fig. 2.

Typical instrumented scratch data: vertical positions of the tip during the pre-scan (C₁, during the scratch (C₂), during the post-scan (C₃) and applied normal force during the scratch as a function of scratch distance. The image above the scratch traces is the corresponding scratch pattern after the test aligned with scratch distance. This example was performed by macro-scratch test on sample C3.

The estimated uncertainties of quantities presented in this paper are at one standard deviation from the mean value from at least three scratch tests.

2.2.1. Nano-scratch test

A nano-scratch test (ASTM D 7187-15) [14] was performed on all clearcoats using a nano-scratch tester (NanoXP depth sensing instrument, Keysight technologies) with a sphero-conical diamond tip of 1 μm radius with a 90° apical angle. During the nano-scratch test, a progressive force scratch was applied normal to the coating’s surface. As the sample moved laterally with a velocity of 3.0 mm/min, the normal force was continuously increased from 0 mN to 30 mN with a load speed of 90.0 mN/min to generate a 1 mm scratch length. The pre-scan and post-scan were conducted under 0.1 mN load. The scratch images were used to determine the point of first failure and images were taken with a laser scanning confocal microscope (LSCM).

2.2.2. Micro-scratch test

An instrumented micro-scratch test [15] was performed by drawing a sphero-conical diamond tip with a 60° cone and 50 μm radius at a constant velocity of 4 mm/min across the surface under a progressive load from 25 mN to 5 N over a 2 mm length. During the test, the penetration depth, tangential force and acoustic emission signals are recorded. The test was conducted on an Anton Paar Micro-scratch tester (Anton Paar, MCT) by using the three measurement stages as described above. During the pre-scan and post-scan, the tip was used to profile the surface under a nominal 25 mN load before and after the scratch. Following the test, the full scratch is imaged via the microscope allowing the creation of a panorama that is used in conjunction with the data to determine the points of failure.

2.2.3. Macro-scratch test

A tribometer (Bruker UMT Tribometer) with a sphero-conical diamond tip of 200 μm radius and 120° apical angle was applied to clearcoats to generate macro-scratch patterns. The sample was moved laterally with a velocity of 4.0 mm/min, and the normal force was continuously increased from 0.5 N to 30 N with a loading rate of 23.6 N/min to generate a 5 mm scratch length. A comparison of the three instrumented scratch tests conditions is summarized in Table 2.

Table 2.

A comparison of instrumented scratch test condition from small to large scale.

	Nano-Scratch Test	Micro-Scratch Test	Macro-scratch Test
Indent type	90° sphero-conical	60° sphero-conical	120° sphero-conical
Indenter radius	1 μm	50 μm	200 μm
Initial load	0 mN	25 mN	0.5 N
Final load	30 mN	5 N	30 N
Scratch length	1 mm	2 mm	5 mm
Velocity	3.0 mm/min	4.0 mm/min	4.0 mm/min
Loading speed	90.0 mN/min	9.95 N/min	23.6 N/min

2.3. Scratch characterization

2.3.1. Laser scanning confocal microscope (LSCM)

A reflection laser scanning confocal microscope (LSCM, Zeiss model LSM510 Meta) was employed to characterize scratch morphology. The laser wavelength used in this study was 543 nm. LSCM images presented in this paper are 2D intensity projections (an image formed by summing the stack of images over the z direction) (512-pixel × 512-pixel) of the coating surface. The 2D intensity projection images are effectively the sum of all the light scattered by different layers of the coating, as deep as approximately 100 μm in the 5× magnification configuration. The details of scratch profile characterization are described elsewhere [16].

2.3.2. Optical confocal microscope

The scratch patterns generated by the macro-scratch test were studied by using an Optical Confocal Microscope VHX5000, Keyence with a 150x objective lens.

2.4. Industry standard scratch tests

2.4.1. AMTEC-Kistler test (wet scratch)

The test (Standard ISO 20566) utilized a polyethylene brush drum that was rotated over the surface of the samples. As the panels moved beneath the brush, the brush rotated the bristles against the direction of sample movement. A quartz dispersion (1.5 g silica per liter of water) was sprayed during the test to simulate dirt and other abrasive particles. The samples were exposed to 10 cycles in the test apparatus to simulate carwash scratching. The initial gloss and residual gloss were measured before and after Amtec-Kistler test, respectively.

2.4.2. Thermal healing process

After collecting residual gloss values, all the panels scratched by Amtec-Kistler test were consequently treated in an oven at 60 °C for 2 h. After cooling, the gloss measurement was conducted and recorded as ‘residual gloss after healing’.

2.5. Mechanical and thermo-mechanical tests

2.5.1. Uniaxial tensile test

Tensile testing was carried out to generate stress-strain curves for free-standing films with 0.25-inch (6.35 mm) width at 23 °C and 50% RH to 55% RH (RSA-G2, TA Instruments). The cross-head speed (0.0833 mm/s) provided an effective strain rate of 0.5 s⁻¹.

2.5.2. Dynamic mechanical thermal analysis (DMTA)

DMTA testing was performed on free-standing films with 0.25-inch (6.35 mm) width using a Q-800, TA Instruments. The test was performed under tensile loading from −50 °C to 200 °C, using a strain of 0.04, frequency of 1 Hz. and temperature ramp of 3 °C/min.

3. Results and dicsussion

3.1. Viscoelastic and mechanical properties

Scratch performance is often influenced by the mechanical properties and viscoelastic nature of coatings [17]. To better understand the correlation, uniaxial tensile tests and DMTA have been performed on free-standing clearcoat films prepared as described in Section 2.2. The resulting mechanical properties through stress-strain curves for 1 K melamine formaldehyde and 2 K PU clearcoats were summarized in Table 3. The characteristics of the plastic or elasto-plastic deformation regions such as yield, strain-at-break etc. often play a more crucial role when considering mechanical insights for scratch resistance. Table 3 shows that both commercial clearcoats B and C exhibit failure at approximately 2% strain, indicating the crack starts to grow and the crack propagates rapidly to the other side of the specimens. In comparison to B and C, sample A shows the lowest modulus, but relatively higher strain-at-break. Regardless of specific strain-at-break value, it has been found that A, B, and C exhibit relatively low strain-at-break with no yielding behavior before failure. In contrast, clearcoat X continues to deform beyond yield to a strain-at-break of approximately 8%. In this study, yield was determined by the first derivative of stress with respect to the strain is equal to zero and the second derivative is negative in a stress-strain curve. The area under a stress-strain curve (toughness) is the total energy absorbed by a material prior to failure [18]. Clearcoat X has a dramatically more ductile character than the other polyol-melamine formaldehyde clearcoats and it resembles the deformation of the PU clearcoats. This step change in film ductility without compromising modulus or yield strength is unique among polyol-melamine formaldehyde clearcoats, and this correlates with differentiated performance in the scratch methods described below.

Unlike in most melamine formaldehyde-cured clearcoats, yielding is observed in all the PU clearcoats tested here (Table 3 and Fig. S2b). This trend suggests PU clearcoats may generally exhibit higher levels of ductility as compared with melamine formaldehyde-cured chemistries. Y and Z clearcoats possess increased yield stresses (which is the ultimate tensile stresses in this case) as compared with clearcoat D. However, a ratio to consider is E/σ_y, where E is Young’s modulus and σ_y is the yield stress (Table 3). This ratio decreases moving from D to Y and Z. Additionally, all the PU clearcoats show a lower E/σ_y ratio as compared to clearcoat X, which was the only polyol-melamine formaldehyde clearcoat that yielded. The general methods identified in literature to improve scratch resistance from a mechanical standpoint are either (i) through decreasing the E/σ_y ratio or (ii) through introducing a strain-hardening effect [19,20]. A decrease in E/σ_y coincides with an increase in the elastic component in an elasto-plastic deformation, thereby decreasing the material sensitivity to scratch. One of the common risks of using this approach is the possible sacrifice of Young’s modulus with subsequent loss of hardness [19]. However, T’ and T” in PU clearcoats were able to achieve lower E/σ_y ratios while maintaining comparable or even higher Young’s modulus relative to the commercial system.

The viscoelastic characteristics of the clearcoats probed using DMTA on free-standing films are summarized in Table 4. In Table 4, the glass-transition regions of all the clearcoats are found to be reasonably above room temperature. Therefore, all of them are glassy when tested by scratch and tensile tests at room temperature. When considering scratch resistance and recovery, some authors [2] have found that significant improvement can be achieved by significantly reducing the film T_g (or tan δ peak) down to the test temperature. However, this approach can lead to trade-offs in chemical resistance, acid etching resistance or other barrier properties, and it increases the risk of failure when these low T_g coatings are exposed to environmental conditions such as acid rain, tree sap or bird droppings.

Table 4.

Viscoelastic properties of the clearcoats determined by DMTA testing. CLDs were calculated using storage modulus (E’) at 160 °C: CLD = E’/3RT, where R is gas constant and T (°K) is thermodynamic temperature [2]. The uncertainties are standard deviation from five replicates. The original DMTA curves are shown in Fig. S3a–d.

Sample name	Tg (°C) determined by tan δ peak	Tan δ peak FWHM (°C)	Cross-linking density (mmol/cm3)
A	91.1 ± 0.8	47.2 ± 0.3	2.71 ± 0.07
B	96.9 ± 0.9	44.3 ± 0.3	4.21 ± 0.14
C	95.3 ± 0.9	50.0 ± 0.4	3.90 ± 0.13
X	85.3 ± 0.5	73.6 ± 0.5	4.33 ± 0.12
D	76.5 ± 0.4	27.6 ± 0.2	1.59 ± 0.03
Y	96.6 ± 0.8	22.0 ± 0.1	1.54 ± 0.02
Z	85.3 ± 0.5	22.9 ± 0.1	1.46 ± 0.03

A greater breadth of the α-transition, measured as the full width at half maximum (FWHM) of the tan δ peak, is observed for the clearcoat X as shown in Table 4. PU clearcoats in this study have significantly sharper tan δ peaks than the polyol-melamine formaldehyde systems. In general, the heights of tan δ peak is related to the strength and number of relaxation processes while breadth is associated with the differences in activation energy due to differences in local mobility [21–23]. This analysis suggests that the heterogeneity of the polyol-melamine formaldehyde clearcoats is greater with fewer or weaker relaxation processes as compared to PU clearcoats. Compared to clearcoat D, both clearcoats Y and Z have higher T_gs with higher and sharper tan δ peaks. Clearcoat X has a higher cross-linking density (CLD) calculated from plateau modulus in the rubbery region as compared with clearcoats A, but it is comparable to clearcoat B and C.

3.2. Nano-scratch and industrial tests

Mar performance of automotive coatings can be complex as many variables are involved in the inspection, such as basecoat color, lighting, duration of inspection, size or type of damage [11]. To quantitatively study mar performance, the nano-scratch properties of each layer-by-layer sprayed coating system were measured by a progressive-load scratch method with tip size of 1 μm. The values of penetration depth, residual depth, scratch recovery and fracture resistance are reported in Table S1.

LSCM images of the ends of scratch patterns (near 30 mN) generated by the test on each clearcoat surface are given in Fig. S4. The typical fracture or rupture damage characteristics have been observed on all the samples, however the severity of damage can still be indicated by the length and size of the side cracks. Considering the variables such as clearcoat cross-linking chemistries and basecoat types, the comparison of scratch morphologies was made with the respect to clearcoat and basecoat types: the clearcoat X (or Y and Z) with basecoat 1–4 was able to reduce the propagation of side cracks at the fracture region. To quantitatively describe the resistance to fracture, the load at the starting point of each fracture is recorded as fracture resistance (as per ASTM D7187).

In addition to fracture resistance, plastic deformation identified by residual depth (Rd) is often used to indicate mar resistance and correlate with carwash results, and it has shown strong correlation with mar performance [7,24]. Considering both metrics as a whole, a more comprehensive way to study this performance is to plot them as shown in Fig. 3a. The plastic deformation was determined by Rd after a 5 mN load is applied. The best performing clearcoats exhibit the combination of the smallest amount of plastic deformation and the highest resistance to first fracture or tear.

Fig. 3.

(a) Plastic deformation at 5 mN vs. fracture resistance with a tip size of 1 μm; error bars represent calculated standard deviations from measurements on three different samples. (b) 20° gloss retention from carwash test as a function of plastic deformation at 5 mN obtained from the nano-scratch test; error bars represent calculated standard deviations from two samples and nine readings on each sample. The measured values shown in (a) and (b) are given in Table S1 and Table S2, respectively.

In Fig. 3a, it is found that the clearcoats X, Y and Z were able to consistently achieve higher fracture resistance on various basecoat systems compared to the commercial clearcoats. The fracture resistance in commercial systems shows that B2 and D4 have better performance than C3 and A1; while the plastic deformation from low to high can be ranked as: B2, C3, A1, D4, corresponding to best to worst performance (Table S5). For all the samples, plastic deformation only occurred on the top surface (1 μm–2 μm depth, as shown in Table S1) of the clearcoat, however plastic deformation behavior could be complex. It is highly dependent on scratch recovery which is usually associated with the surface elasticity of the clearcoat in service.

As a well-established industrial test method, the Amtec-Kistler carwash is often performed to measure the scratch resistance under simulated car washing circumstances. 20° gloss measurements were conducted before and after the Amtec-Kistler carwash test, as well as after a 2 h thermal healing process at 60 °C (Table S2) to determine the overall carwash performance. Fig. 3 shows that lower plastic deformation at 5 mN as determined by the nano-scratch test typically leads to higher gloss retention in the Amtec-Kistler carwash test. With the increase of plastic deformation, the reductions of carwash performance before and after a thermal healing process follow two stages: (i) when plastic deformation < 400 nm, the 20° gloss retentions show little or no change; (ii) however, when plastic deformation > 400 nm, the 20° gloss retentions decrease significantly with the increase of plastic deformation. As discussed above, it is believed that there is a correlation between Amtec-Kistler carwash and nano-scratch test. In this particular study, plastic deformation < 400 nm under a 5 mN load is a critical threshold to ensure high gloss retention values (85% ± 5%). In addition, it is found that PU clearcoats exhibited higher percentage of thermal recovery (Table S2) during the thermal healing process as compared with polyol-melamine formaldehyde clearcoats. It may be due to (i) higher cross-linking density (Table 4) and (ii) lower strength of relaxation processes (tan δ peak heights in Fig. S3b and d) that the polyol-melamine formaldehyde clearcoats have in this study.

3.3. Micro- and macro-scratch tests

Although nano-scratch testing is necessary to understand the mar resistance of the clearcoats, most real-world scratches on vehicles are large, fracture type scratches with width >100 μm created by severe contacts, such as fingernails, car keys, tree branches [7]. Additionally, the thickness of clearcoats requires larger and deeper scratches to investigate their intrinsic resistance to scratch and their possible failure from the basecoat.

By increasing the load to 5 N on a larger diamond stylus, the micro-scratch technique with 50 μm tip allows going beyond the mar resistance to a broader comparison with the more severe damages that could be seen on vehicles. The plastic deformation determined as the residual depth for each sample at 400 mN was plotted against fracture resistance in Fig. 4a. To simulate even larger macro-scratches that occur in the field, tribometer measurements with the tip size of 200 μm were also applied. Fig. 4b displays the corresponding plastic deformation at 5 N is plotted versus fracture resistance for each system.

Fig. 4.

(a) Plastic deformation at 400 mN vs. fracture resistance with a tip size of 50 μm; (b) plastic deformation at 5 N vs. fracture resistance with a tip size of 200 μm. Error bars represent calculated standard deviations from measurements on three different samples. The measure values for (a) and (b) are given in Table S3 and S4, respectively.

The performance ranking based on plastic deformation and fracture resistance obtained from nano-, micro- and macro-scratch tests was summarized in Table 5. Comparing results obtained from all three instrumented scratch tests, the overall trend shows certain consistency with fracture resistance. For example, similar to the way that X, Y and Z performed in nano-scratch test, they maintain the trend of higher fracture resistance relative to the A, B, C, and D clearcoats over their respective basecoats at the larger scales with tip sizes of 50 μm and 200 μm.

Table 5.

Performance ranking based on the nano-, micro- and macro-scratch tests. The mean values were used for ranking. Basecoat 1 - black front; basecoat 2 – red front; basecoat 3 – blue front, basecoat 4 – green front. (For interpretation of the references to colour in the table, the reader is referred to the web version of this article).

However, it is complex to compare plastic deformation for individual samples in all scales of scratch tests. A good performance on plastic deformation from one scale may not always translate to another scale. Therefore, it may be necessary to evaluate the scratch performance of a clearcoat at more than one size scale to understand the performance variation on surface scratch versus deep scratch with the better understanding the scaling behaviors of the scratch test associated with coating system.

As larger and deep scratch performed by macro-scratch test, visible scratch patterns with 5 mm length achieved over 500 μm width and broke through both clear- and base-coat layers at the end of the scratch. Fig. 5a–e demonstrate the two entire scratch patterns on polyol-melamine formaldehyde clearcoats over WBBC (Basecoat 3) generated by macro-scratch and the surface topography was consequently captured by an optical confocal microscope. In Fig. 5a, C3 shows several regimes of deformation from elastic to plastic with increasing scratch severity, and finally to fracture and delamination from the substrate. As the increase of normal load proceeds, a scratch pattern progressing from left to right consists in (i) fully recoverable elastic deformation, (ii) partially recoverable plastic deformation, (iii) fracture (including tearing) and (iv) delamination. These phenomena are similar to what was observed in the micro-scratch test with the smaller 50 μm tip in Section 3.4. With large deformations, it is observed that both C3 and X3 go through elastic and plastic deformations at an early stage of the scratch test. However, the cracking quickly started on C3 after plastic deformation, and was followed by severe clearcoat delamination, whereas X3 was able to withstand much higher load (and strain) before cracking. Considering the stress-strain characteristics presented in Section 3.1, increasing the strain-at-break can provide improved performance for a coating subjected to large deformation scratches, where a fracture type scratch is typically observed. Compared with failure characteristics shown in polyol-melamine formaldehyde C3 (Fig. 5b), one may note that D4 displays overall better resistance to fracture and delamination. These characteristics in 2 K PU chemistry are attributed to the relatively higher strain-at-break of these coatings described in Section 3.1. Consistent with the observations in nano- and micro-scratch tests, Y4 and Z4 also demonstrate drastic increases in fracture resistance compared to D4.

Fig. 5.

The images of confocal optical microscope demonstrated scratch patterns of (a) C3, (b) X3, (c) D4, (d) Y4 and (e) Z4 through macro-scratch test with a tip size of 200 μm. The scale bar is 500 μm.

3.4. Scaling behaviors and stress localization

Although there are some trends across the coatings in this study, the scratch behavior is complex in response to a broad range of loads and tip sizes. Many different reasons can be proposed to explain the differences among the performance of coatings during nano-scratching, micro-scratching and macro-scratching. For the larger tip sizes, it is believed that the underlying layers (basecoat/primer) and interfaces play key roles in the coating performance [7], whereas the mechanical properties of a coating surface might be relatively more important to performance in nano-scratching. For these reasons, clearcoats are typically optimized to coat a specific basecoat, and simple comparisons of clearcoats over different basecoats can be misleading. Comparing the results obtained from instrumented scratches at different scales, the uniformity of the properties of the clearcoat through the thickness needs to be taken into consideration. Intentionally or not, the surface of a clearcoat could possess a different level of scratch resistance than that of the bulk, particularly in some clearcoat formulations that have nanoparticles, surface active components, or other compositional gradients involved [25]. In addition to the above scenarios, it is believed that cross-linking density plays an important role in scaling behaviors associated with stress localization. In this study, the ratio of penetration depth to contact width (Pd/2 r, as shown in Fig. S5) were calculated and utilized to represent the extent of stress localization at given loads with the respect to tip sizes. The 1 μm tip used in nano-scratch gives Pd/2 r value of approximately 0.5, while the 50 μm tip used in micro-scratch leads to lower Pd/2 r values in a range of 0.14–0.19, and the 200 μm tip used in macro-scratch test shows the lowest Pd/2 r values in a range of 0.11–0.13 (Table S5). Based on this calculation, it is found that greater stress localization has occurred with a smaller tip in this study. As shown in Fig. 6a–c, increasing cross-linking density results in an improvement of scratch recovery with all three tip sizes. With reduced tip size, the increased slope indicates that increasing cross-linking density as a way to mitigate damage is more effective on stress localization dominated scratches, such as nano-scratches or carwash scratches.

Fig. 6.

Dependence of scratch recovery on cross-linking density of coatings with tip size of (a) 1 μm, (b) 50 μm and (c) 200 μm. Error bars represent one standard deviations from 3 measurements on each coating. The dashed line is fitted by simple linear regression.

4. Conclusions

In this study, different scratch conditions ranging from plastic grooving to cracking and delamination with sizes from nano-scale to hundreds of microns were systematically investigated through instrumented scratch tests including nano-scratch, micro-scratch and tribological scratch tests on a series of automotive 1 K melamine formaldehyde and 2 K PU clearcoats. Instrumented scratch performance was analyzed in combination with standard industrial scratch test, the Amtec-Kistler carwash, as well as mechanical property measurements: tensile testing and DMTA. These experimental results are summarized below:

4.1. Scratch behaviors and the mechanical/viscoelastic properties

For plastic deformation-dominated mar damages, the nano-scratch test was applied and found to correlate with Amtec-Kistler carwash results. For larger fracture or even delamination type scratches, micro- and macro-scratch tests with larger tips and larger loads were applied to simulate scratches caused by severe contacts. The results from all three tests are consistent on a rank-order basis with the same basecoat (Table 5). Clearcoats X, Y and Z were able to consistently achieve higher fracture resistance on various basecoat systems in all the scales. However, it is found that scaling behavior, particularly on plastic deformation, is associated with stress localization, where greater stress localization occurs with a smaller tip size. Although increasing cross-linking density leads to improved scratch recovery regardless of tip size, it is more effective on stress localization dominated scratches such as mar damages caused by carwash, instead of larger and deep scratches.

4.2. Clearcoat comparisons

From stress-strain curves, all the PU clearcoats were able to deform beyond yield, and they exhibited higher ductility as compared to polyol-melamine formaldehyde clearcoats. From DMTA testing it is found that heterogeneity of polyol-melamine formaldehyde clearcoats is greater with higher cross-linking density, but weaker relaxation processes are observed in the polyol-melamine formaldehyde clearcoats compared to PU clearcoats. Due to relatively lower cross-linking density, PU clearcoats show higher plastic deformation (lower scratch recovery) in all instrumented scratch tests and lower immediate gloss retention after the carwash test. However, the strong sub-T_g relaxation in PUs enables significantly greater recovery of gloss relative to polyol-melamine formaldehyde systems after a thermal healing process. Clearcoats that enabled deformation beyond yield in tension and exhibited significantly higher strain-at-break, toughness and cross-linking density demonstrated consistently reduced plastic deformation as well as improved fracture resistance and delamination resistance in all instrumented scratch tests.

5. NIST disclaimer

Certain instruments or materials are identified in this paper in order to adequately specify experimental details. In no case does it imply endorsement by NIST or imply that it is necessarily the best product for the experimental procedure.