Tribological effects of plasma immersion ion implantation heating treatments on Ti–6Al–4V alloy

Abstract

In order to achieve improved surface in components with high reliability required by the tool or metallurgical industries by means of plasma immersion ion implantation (PIII), it is necessary to obtain quite thick treated layers with reasonable thickness uniformity. One effective way to achieve such a thick nitrogen-rich layer in Ti–6Al–4V alloy is to run PIII process in high temperatures of about 800 °C. In these experiments, we heated the sample-holder and subjected the Ti–6Al–4V samples to nitrogen glow discharge PIII with relatively low peak voltages (5 kV). We also treated the Ti–6Al–4V samples by the traditional PIII method, increasing slowly the high voltage pulse intensities, and at the end of processing, reaching temperatures of up to 350 °C. These modes of treatments were compared with respect to nitrogen implantation profiles, tribological and mechanical properties. Nitrogen profiles measured by AES showed that auxiliary heating leads to richer and deeper layers. The best results were obtained for the samples treated for 4 h that, after 25,000 cycles in unlubricated pin-on-disk test, presented very small wear (reduction of 89%). In spite of friction coefficient being higher in the beginning of the test, probably due to the increase of the average roughness (Rms) in the surface of the treated material, it decreases when the rough surface wears away, due to the reduction of frictional force necessary to slide the pin on the sample. The material treated at 800 °C presents high wear resistance, which is desired in this alloy, that normally possesses excellent mechanical properties, however, poor tribological properties.