Development of copper-alumina composites for abrasive wear applications

摘要

Copper-alumina composites were developed for testing in abrasive wear applications. The composites featured a porous continuous ceramic-preform network infiltrated by a liquid metal to form the final consolidated composite. The liquid metal phase was pure copper.Six different ceramic preform variants were tested. Ceramic volume fractions of 40, 50 and 60% were used, of two preform types; one pure-alumina, and one with additional 2wt% copper(I) oxide (CU20), functioning as an infiltration aid, the effects of which were determined in a previous study; the copper-oxide reduced infiltration pressure and allowed the use of higher ceramic phase volume fraction in the final composite.Abrasive wear tests against two automotive braking system materials were conducted. Grey cast iron of alloy type GG15 was used to establish a baseline for behaviour of the six different composite samples and compare them. Following this, the three volume fraction variants of samples using the copper-oxide infiltration aid were trialled against a commercially-available European passenger vehicle brake pad friction material; ABEX 6091.Wear tests were conducted on a pin-on-disc tribometer. Hemispherical-headed pins were made from the composite and tested against rotating discs of the grey cast iron and the ABEX friction material. Contact velocity was kept constant at Ims-¹ at room temperature in air, and contact loads up to 15N were used. Test loads of 1-4N were used against grey cast iron, and 15N against the ABEX friction material.Optical micrography was used to monitor the wear rate of samples tested against grey cast iron. Scanning electron microscopy (SEM) was used to characterise bulk microstructures and evaluate surface wear features. Transmission electron microscopy (TEM) was used for further microstructural investigation of the sintering and interfacial features of the undamaged pin samples, as well as damage zones and tribofilm compositions. Focussed ion beam (FIB) milling was used to create subsurface cross-sections of wear regions and prepare TEM samples.The wear performance of the different sample types was compared by ceramic content and preform additives. It was found that the wear resistance of pure-alumina preform composites was dependent on ceramic volume fraction. Increasing ceramic content lead to increased wear resistance. The lower sinter temperature of the samples with the copper oxide additive led to reduced wear resistance compared with the monolithic alumina preforms and changes in ceramic volume fractions were not discernable in wear resistance against grey cast iron. This could be further supported by qualitative micrographic observations. All tests against grey cast iron were dominated by tribochemical film formation, which was determined to be oxidation of the iron which formed at the composite pin contact surface.Further testing of the copper-oxide containing samples against the ABEX friction material revealed a mixed resu the 50 and 60% ceramic volume samples produced near-identical wear performance, while the 40% sample suffered poor wear resistance. The dominant wear mechanism of composite pins tested against the ABEX friction material was abrasive wear. Sub-surface analysis of wear pins revealed a prominent damage layer forming at the contact surface of all pin samples which progressively grew into the bulk material. This layer was believed to have an important effect on the wear behaviour of the materials.