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They vary in a step function manner, that is to say they have similar Lc values for under 20 minutes pre-sputtering time, then the values suddenly increase for 20 to 35 minutes pre-sputtering time, but after 35 minutes, further increasing the time has insignificant effect on

Figure 5 shows that the wear rate of the coating decreases when the pre-sputtering time increases. In figure 5 (a), the wear rate initially decreases from 2.5xl0"15 to 1.8xl0·15 m3/Nm when increasing the pre-sputtering time from 3 to 20 minutes, the wear rate drops suddenly to less than 0.5xl0·15 m3/Nm when the time increased beyond 35 minutes. Further increase in time to 60 minutes has little effect on the wear rate. The coatings on non-hardened steel substrates show different behaviour. Figure 5 (b) shows that the wear rates decreases gradually from 10 xl0"15 to 9 xi0-15 m3/Nm for the same pre-sputtering time range. It is believed that ion cleaning removes the contaminants on the surface of the substrate and hence strengthening the interfacial bonds between coating and substrate; therefore the coating provides better wear resistance.

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Pre-sputtertng Time (min)

Fig. 5. Wear Rate versus pre-sputtering time (a) hardened steel substrate (b) non-hardened steel substrate

is less than 35 minutes, even increasing the time will not have significant effect on the interface between coating and substrate. When the sputtering time reaches 35 minutes, pre-sputtering on the substrate greatly enhances the strength of interfacial bonds between coating and substrate so that the coating has lower wear rate and higher critical load. After 35 minutes, as the substrate has reached its optimal clean level, further increase in pre-sputtering time will not improve the substrate surface quality much. Therefore, no correlation between wear and adhesion is found in group C also.

In each group of data in figure 9 (a), the data also scatter to a large extent which has variation about 10 N in critical load. The correlation between wear and adhesion is poor if we focus on each group of data individually. As many factors will affect the testing results especially in wear tests, such as deformation and temperature, variation of the data is expected and acceptable.

Therefore, if the range of the critical load and wear rate results in the experiment is not large enough, it is possible that the relationship cannot be demonstrated. As the change of the coatings or any other experimental details will affect the results, the range of the variation may be different in other cases. However, it shows that if the range of indenter load is too narrow (say 20N), a poor correlation will be found.