A Ni3Al-alloy and its Composites as Potential Wear Resistant Materials for Advanced Applications

Doctoral thesis, 2011

The Ni3Al-based intermetallic alloys as engineering materials in service, they may be subjected to a wear environment of some sort. Hence, it is technologically important to understand the tribological response and to determine methodologies to improve wear resistance of these advanced materials. The tribological performance of Ni3Al-based alloys is among the mostly promising in the group of intermetallics. Therefore, a Fe-alloyed Ni3Al (NAC-alloy) with composition of Ni-18.8Al-10.7Fe-0.5Mn-0.5Ti-0.2B (at.%) and its composites tailored respectively with hard Cr3C2 and soft MnS particles were selected in this thesis work for investigation. The testing materials were prepared by powder metallurgy, vacuum casting process and cladding technique, respectively. To evaluate the friction coefficient and specific wear rate of the test materials, a conventional pin-on-disk (POD) tribometer was used. The monolithic NAC-alloy showed similar wear properties with respect to its specific wear rate and friction coefficient under the applied pin-on-disk testing conditions to those of commercial vermicular graphite cast iron. Unfortunately, NAC-alloy worked inadequately with its counterpart disk of a grey cast iron used as liner material in engines. Added hard Cr3C2 particles reduced wear on both sides of the pin and disk by 50%. The soft MnS particles, which functioned as an effective solid lubricant, reduced friction coefficient of the friction pair and decreased specific wear rate of the grey cast iron disk by 20%. However, the specific wear rate of the pin was not lowered. In general, the NAC-alloy may function well as a matrix material to develop wear-resistant composites. In this thesis study, it was also recognized that development of metallurgical processes may play an important role for further improving wear properties of the investigated materials. For further understanding wear mechanism of the NAC-alloy and its composites, nanoindentation and numerical simulation were applied to study friction-induced nanohardness variation in subsurface layer of the tested materials. An exponential decay fitting function was applied and the deformation strain of the monolithic NAC-alloy versus surface distance was derived. The addition of hard Cr3C2 particles reduced the thickness of the subsurface layer and retained the same peak hardness of the friction surface as the monolithic NAC-alloy. Due to ineffective strain hardening, the addition of soft MnS particles decreased both of the thickness of subsurface layer and the peak-hardness on the friction surface. These results were coupled to the experimental data from pin-on-disk tests and microstructural analysis.