Tribological behaviour of ultrafine-grained alloys formed by severe plastic deformation

Abstract

This thesis presents a study on wear behaviour of materials processed by severe plastic deformation (SPD). Al-1050 alloy and commercial purity Ti have been processed via different SPD methods. Wear tests of different contact size were employed to compare the wear performance between ultrafine grained (UFG) alloys and their as-received state. The aim of this study is to understand the effect of SPD processing on wear behaviour of materials, to seek a way to use SPD processing to improve the mechanical properties of materials and their wear resistance at the same time. Al-1050 was processed using equal-channel angular pressing(ECAP) and highpressure torsion (HPT), respectively. Microhardness evolution was analysed by Vickers hardness testing. The results showed clear evidence of strength improvement by SPD. Afterwards, dry sliding testing was performed on a TE77 tribometer against different counter materials. The results showed that both ECAP and HPT led to a decrease to the wear resistance to Al-1050. Scanning electron microscopy, energy dispersion spectroscopy and surface profile meter were used to examine the worn surface and debris. The transition from severe platelet wear to oxidation wear was observed during the wear tests. The decrease of wear resistance of Al-1050 after SPD processing is attributed to a lack of work hardening capacity during the severe wear stage and a higher oxidation rate during the oxidation wear stage. In addition, commercial purity Ti was processed via HPT and heat treatment. Mechanical testing and a microstructure study demonstrated the trend of increasing strength with decreasing grain size. In this study, the micro-tribological behaviour of UFG Ti was studied for the first time via microscratch testing. The results showed that HPT-processing of Ti inhibited operation of adhesion and ploughing during wear tests and led to better wear resistance. Based on the results from the above studies and a comprehensive review of published research on wear of UFG alloys, a conclusion was drawn that when the wear process is dominated by adhesion andoxidation wear, SPD processing decreases the wear resistance of materials. However, when wear is dominated by mechanical wear mechanisms (plastic deformation, abrasion and ploughing), the strengthening of SPD processing brings better wear performance to the material. To enhance the wear resistance of UFG Ti as bio-implant materials, TiN and DLC coatings were deposited on to Ti substrates via physical vapour deposition methods. Wear tests indicated a significant improvement of wear resistance after coating deposition. Adhesion tests showed that the thin coatings had much enhanced load bearing capacity with UFG Ti as the substrate compared to coarse-grained Ti, which is explained by a modified composite hardness model. This model showed good accuracy in predicting the critical load of a wide range of thin coatings on different substrates. Finally, an improved bio-implant design was proposed for total joint replacement applications. This design involves fabricating the main body of the bio-implant from UFG pure Ti processed by SPD and subsequently applying a hard thin coating to protect the head of the implant. It is anticipated this design will provide the implant with high strength, good fatigue life, good corrosion resistance, together with good wear and tribo-corrosion resistance from the coating and a non-toxic ion release. The product is aimed to replace the toxic bio-metals in total joint replacement use, such as SS316, Co-Cr alloy and Ti-6Al-4V. Therefore, the design has a very strong application prospect.

More information

Identifiers

ORCID for Nong Gao: orcid.org/0000-0002-7430-0319

Catalogue record

Export record