Hardness as a Measure of Wear Resistance
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HARDNESS AS A MEASURE OF WEAR RESISTANCE 1 2 3 W.C. OLIVER 5 , R. HUTCHINGS , J.B. PETHICA , I.L. SINGER 4 , AND G.K. HUBLER 'United Technologies Research Center, E. Hartford, CT 06108; 2Brown Boveri Research Center, CH-5405 Baden, Switzerland; 3 Cavendish Lab., Cambridge, CB3 OHE, U.K.; 5 4, Naval Research Laboratory, U.S.A.
ABSTRACT One measure of the surface mechanical properties of materials can be obtained through microhardness data. The success of microhardness in predicting the improvements in wear resistance of ion implanted metals has been mixed. In this paper the cases of N implantation into 304 S.S. and Ti implantation into 52100 bearing steel will be examined. Microhardness data indicates little or no hardness changes whereas large wear rate changes are observed. From these two examples it is clear that the wear mechanism, the chemical nature of the surface, the ductility, and the toughness can be more important than the hardness changes.
INTRODUCTION
Two tribological systems of technological interest that have been studied extensively are N implanted 304 S.S. and Ti implanted 52100 bearing steel. Much of the early work on these systems involved extensive wear testing and little evidence was collected concerning the mechanism by which the improvements might occur. General explanations involving increased hardness of the implanted surface as giving rise to better wear resistance have been offered. The hardness of a material does affect its wear rate; however, other parameters can be more important. Hardness gives some indication of strength. Other mechanical properties of a material that help determine its wear rate are ductility, toughness and the temperature dependence of these quantities. Hornbogen [1] and Atkins [2] consider the effects of some of these parameters on wear rates. The mechanical structure of the surface is also important. For example, a very hard surface on a very soft substrate does not result in high wear resistance if high contact stresses occur. Finally, chemical effects between each component of the wear couple and the atmosphere and between the two components themselves can also be extremely important. Many of these properties are very difficult to measure. Ease of measurement has led to hardness being used extensively when considering wear experiments and modeling of wear processes. In the case of ion implanted surfaces, even hardness is very difficult to measure.
Mat.
Res.
Soc. Symp.
Proc. Vol.
27 (1984) QElsevier science Publishing Co.,
Inc.
604
A hardness tester that is capable of sampling the very thin implanted layers has been constructed and used to measure the hardnesses of implanted surfaces [3-5]. The results of these tests, wear data, and microanalysis results for a range of implanted metals have been presented [6]. By comparing hardness and wear data, it has been shown that indentation hardness is qualitatively correct in predicting pin on disc wear property improvements. The magnitude of the improvements is much less predictable from the hardness data. In
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