Table 1.

The important properties of coatings for joint implants, their evaluation methods and their target profile.

Property	Target profile	Typical method for evaluation and standards
Hardness	A high hardness will help mitigate wear. However, the stiffness should also be considered as the ratio of hardness to Young's modulus gives a measure of the elastic limit in the contact and hence provides an indicator of wear performance.	Nano- or microindentation. Values are given in Pa or Vickers hardness number (HV).
		• ASTM E2546-07 [58]
		• ISO 14577-4 [59]
Wear resistance	A low wear rate is desirable, but actual values will depend on the specific tribological situation and are therefore not specified here. Attention should be paid to the generated wear debris, the size, shape and volume will likely influence the immune response in the final application.	Tribological set-up ranging for pin-on-disc to joint simulators. The resulting wear is measured as specific wear rate (mm3/Nm) or mass loss per million cycles (mg/Mc).
		• EN 1071-12 [60]
		• EN 1071-13 [61]
		• ASTM F732 [62]
		• ISO 20808 [63]
Corrosion resistance	A coating should protect the underlying metal from corrosion as well as have a low rate of degradation. However, it is also important to consider the character of the particles and ions that inevitably are released.	Measuring open circuit potential (V) and corrosion current (μA).
		• ASTM G5 [64]
		• ISO 16429 [65]
		• ISO 16773 [66]
Toxicity	Ultimately the coating, and more importantly the ions and wear debris, should not elicit an adverse immune response. The toxicity will depend on the volume of debris or ions, i.e. a dose dependency, and the volume will depend on the wear properties of the coating.	In vitro studies using cell lines. Results are given as cell viability.
		• ISO 10993 [67]
Surface roughness	A smooth surface is necessary to reduce PE wear. An Ra value of ≤20 ​nm has been specified for ceramics in ISO 7206-2.	Optical or stylus methods for surface characterization. The most common parameter to report is the average surface roughness, Ra, (nm).
		• ISO 4287 [68,69]
		• ISO 4288 [70]
		• ISO 25178-604 [71]
Adhesion	A coating that adheres well to the substrate is of utmost importance as delamination of the coating could cause excessive wear through the release of abrasive debris. It is important to consider factors such as time and corrosive environments when evaluating the adhesion.	Most common methods are scratch tests, from which critical loads are obtained (N), or Rockwell indentations that are categorized according to a standard.
		• ISO 26443 [72]
		• ISO 20502 [73]