(a) Rapeseed oil and (b) Rapeseed oil with NPs at RT The contact lubricated with mineral oil and NPs additives at RT show lower wear than that of mineral oil only as shown in Figure 5-18. The NPs reduce wear in the tribo-contact by up to 37.4 % (WSW) compared to mineral oil only. NPs can fill the asperity valleys and form tribofilm to support load. More than one NP lubricating mechanism could co-exist in the contact. Figure 519: Optical micrographs (x20) of wear scars on flat friction coupon, after tests with: (a) Mineral oil and (b) Mineral oil with NPs additives at RT Wear Mechanisms on the Ball Sample Although the major wear mechanisms are expected to be observed on the flat sample, the ball also shows surface damage. The optical micrograph of ball after test with a lubricant is therefore necessary. As usual, the harder material in the tribo-pair would experience material transfer, while the softer experience surface material removal. Some ploughed and transferred materials from the softer would bond to the harder sample. These images show some differences in the surfaces of balls with respect to lubricant. The ball in the tribo-pair lubricated with rapeseed oil has some material transfer layer, as shown in Figure 5-19. The material removed from the flat sample, are transferred to the ball. However with rapeseed oil-NPs mixture, the layer spalls off and seem as third bodies, thereby promoting abrasion. This partly explains the slightly higher mean WSW with rapeseed oil-NPs mixture. Figure 520: Typical optical micrographs (x10) of balls with lubricant: (a) Rapeseed oil and (b) Rapeseed oil-NPs mix at 100oC Figure 521: Typical optical micrographs (x10) of ball with lubricant: (a) mineral oil and (b) Mineral oil-NPs mix at 100oC
(a) Rapeseed oil and (b) Rapeseed oil with NPs at RT The contact lubricated with mineral oil and NPs additives at RT show lower wear than that of mineral oil only as shown in Figure 5-18. The NPs reduce wear in the tribo-contact by up to 37.4 % (WSW) compared to mineral oil only. NPs can fill the asperity valleys and form tribofilm to support load. More than one NP lubricating mechanism could co-exist in the contact. Figure 519: Optical micrographs (x20) of wear scars on flat friction coupon, after tests with: (a) Mineral oil and (b) Mineral oil with NPs additives at RT Wear Mechanisms on the Ball Sample Although the major wear mechanisms are expected to be observed on the flat sample, the ball also shows surface damage. The optical micrograph of ball after test with a lubricant is therefore necessary. As usual, the harder material in the tribo-pair would experience material transfer, while the softer experience surface material removal. Some ploughed and transferred materials from the softer would bond to the harder sample. These images show some differences in the surfaces of balls with respect to lubricant. The ball in the tribo-pair lubricated with rapeseed oil has some material transfer layer, as shown in Figure 5-19. The material removed from the flat sample, are transferred to the ball. However with rapeseed oil-NPs mixture, the layer spalls off and seem as third bodies, thereby promoting abrasion. This partly explains the slightly higher mean WSW with rapeseed oil-NPs mixture. Figure 520: Typical optical micrographs (x10) of balls with lubricant: (a) Rapeseed oil and (b) Rapeseed oil-NPs mix at 100oC Figure 521: Typical optical micrographs (x10) of ball with lubricant: (a) mineral oil and (b) Mineral oil-NPs mix at 100oC