The effect of base stock and additive packages in an automotive oil on the friction and wear behaviour of overhead camshaft and finger follower systems
The introduction of overhead camshaft valve train systems some 25 years ago has resulted in improved engine performance and efficiency, this in turn however, has caused increased wear problems. In order to understand the tribological behaviour of overhead camshaft systems, test engines and dynameter test methods have been extensively used. These are however, time consuming and expensive, a cheaper and quicker method of camshaft material and lubricant appraisal is therefore required and this has resulted in the design and construction of a simulative overhead camshaft test rig. A range of commercially available camshaft and follower materials have been tested using both a fully formulated and a -mineral base -equivalent. The most commonly encountered cam and follower wear failures of scuffing, pitting and polishing were reproduced. Results showed scuffing wear to be associated with all of the camshaft materials using the mineral base oil under high load, boundary lubrication conditions. Scuffing also occurred under low load, mixed lubrication conditions using the case-hardened steel cam and chilled white iron follower combinations. The remaining cam materials, carbonitrided and induction hardened grey flake iron did not fail under similar test conditions due to the presence of free graphite in the icrostructure. The fully formulated oil generally suppressed the onset of scuffing under all test conditions due to the presence of the zinc-dialkyldithiophosphate (Z.D.D.P) anti-wear additive in the oil. Pitting failure was associated with the chilled white iron follower using the fully formulated oil under high load conditions. This failure could have been initiated by either a surface, sub-surface or stress - corrosion mechanism. Analysis of this type of failure is complicated by the difference in both chill depth and hardness and also the presence. of cracks within, samples prior to examination and testing. Standardisation of chilled white irons therefore needs to be carried out in order to understand fully the wear behaviour of such materials. Polishing wear was associated with the fully formulated oil at low loads, and appeared to occur by a chemical reaction between the contacting surfaces and the additives present in the oil. The use of the acetate replica technique proved a satisfactory method of analysing the change in surface topography of cam surfaces. Using the fully formulated oil the cam surface 'ran in' satisfactorily, by a process of plastic deformation and fracture, whilst the mineral base oil caused instanteous scuffing of the cam surface. Once the surfaces had 'run in' satisfactorily by using either the additives in the oil or by a diamond lapping operation, the cams continued to run without scuffing in the mineral base oil under continuous, non stop test conditions. An intermittent, stop/start operating cycle however results in eventual scuffing due to additive film depletion and repeated surface interaction under boundary lubrication conditions. From a range of 'new' materials tested for follower application the ceramics, with the exception of the toughened zirconia, exhibited the best wear characteristics using a case hardened steel cam as a standard and tested under mineral base oil conditions. The silicon carbide and sialon ceramics both exhibited a low frictional resistance and wear rate, whilst the metallic follower materials of cobalt and nickel base hard facing alloys and a ceramic fibre reinforced aluminium/silicon alloy all suffered some form of material.