Ni-Cr based Self Lubricating Composites with the Different Solid Lubricants Produced through Powder Metallurgy Method

WAN FARHANA, MOHAMAD (2021) Ni-Cr based Self Lubricating Composites with the Different Solid Lubricants Produced through Powder Metallurgy Method. PhD thesis, Universiti Malaysia Sarawak.

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Abstract

This work covers this main following aspects: process of Ni-Cr based self lubricating composites with and without the addition of solid lubricants through the powder metallurgy method. Then, study on the effect of sintering temperature and atmosphere on the physical and mechanical properties of the composites. Furthermore, investigation of the effect of single, dual and multiple solid lubricants additions on the tribological properties of Ni-Cr based composites is also studied. Self lubricating composites consist of Nickel (Ni), Chromium (Cr) and molybdenum disulphide (MoS2), silver (Ag) and calcium fluoride (CaF2) as solid lubricants. The powder mixture was mixed, compacted and sintered at 1000 oC, 1100 oC and 1200 oC for 1 hour. The sintering atmosphere is also varied with (i) flowing argon and (ii) vacuum then purging through argon. The results showed the densification of Ni-Cr based composites increased from (65%-73%) to (81%-85%) as the sintering temperature increased from 1000 oC to 1200 oC. Most of the composites achieved the highest hardness (112 HV-148 HV) at sintering temperature of 1200 oC with the addition of 5% of each solid lubricant (F1). By optimizing the sintering temperature, the maximum value of hardness was achieved with the least porosity and higher densification. Applying a vacuum before purging the argon gas resulted in some improvement in the relative density at about 3-7%. XRD analysis proved the existence of solid lubricant/s after sintering process. The friction tests were conducted with a ball on disc configuration. The friction coefficient and wear rate were measured by using Alicona surface profilometer and worn surface were observed under SEM. The results showed that MoS2 as solid lubricant provided the best lubrication at room temperature, demonstrated by a low friction coefficient compared to pure Ni-Cr composites (0.176 compared to 0.394). Dual and multiple additions of solid lubricants were also able to reduce the friction of coefficient to 0.202 and 0.31 respectively compared iv to pure Ni-Cr (0.394) composite. SEM analysis also showed that the spreading of solid lubricant/s during the sliding test to provide lubrication to both contacting metals is apparent on certain worn out surfaces. Dual and multiple additions of solid lubricant resulted in the lowest wear rate (2.7 x 10-4 mm3/N.m and 3.47 x 10-4 mm3/N.m) compared to pure composite (4.78 x 10-4 mm3/N.m). As a conclusion, the composites achieved higher hardness and better densification after sintering at 1200 oC with the vacuum and argon gas atmosphere. The composites also exhibited less than 20% porosity and homogeneous structure. With the maximum hardness of the composites, MoS2 added in the Ni-Cr based composite attributed the lowest friction coefficient at room temperature test. Dual addition of solid lubricants, NCMCf achieved the lowest wear rate (2.7 x 10-4 mm3/N.m) and low friction coefficient (0.202) with maximum hardness (113 HV) is the best combination for the room temperature application. The results are significant that ensure the ability of the composite to reduce friction and wear during the start-up of the operation. The composites also can be potentially applied in the wide range temperature application.

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