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Comments on “The delamination theory of wear”
Wear, 47 (1978) 417 - 419 0 Elsevier Sequoia S.A., Lausanne -Printed
417 in the Netherlands
Letters to the Editor Comments on “The delamination theory of wear”
N. P. Suh [l] has identified a wear process that had not previously been observed and has vigorously pursued the phenomenon from a number of viewpoints. The large number of papers covering the subject will stimula~ a new awareness in what must be described as “wear mechanics” and will certainly lead to a higher level of understanding of the wear process. However, two overriding questions must present themselves to any researchers studying the plethora of papers on the delamination theory. Firstly, what is the real essence of the delamination theory, i.e. what are its bounds, and, secondly, how relevant is the theory to one’s own particular area of interest ? A general answer to the first question appears to be that the delamination theory is relevant to situations where the wear rate is determined by the mechanical properties of the materials concerned rather than by effects such as oxidation. This leads to the second question. The successful expe~ment~ evidence suppo~ng the theory has been obtained from wear experiments performed in inert atmospheres. The gener~ization of the theory to a normal situation where air and other surfactants are present has not been proved. It has already been well established that crack initiation can occur either through plastic failure of material at high subsurface stress levels or stress concentration caused by inclusions. The former is the basis of many wear theories [2] , whereas the latter has been studied extensively in connection with roller bearing fatigue failure [ 31. The question is whether, as postulated in the delamination theory, this process of crack initiation and propagation is related to the wear rate. Here the familiar catagorization of “mild” and “severe” wear should be considered. Mild wear encompasses the majority of wear processes where the wear debris has a high oxide content. It does not only apply to high speed sliding as implied by Suh. In mild wear the oxide formed on the surface of the metal prevents the formation of a homogeneous deformed layer and favours the formation of small surface cracks. Both effects result in a situation that is so dominated by surface oxidation processes that the delamination theory can no longer be applied. Lubrication adds to the complexity of the chemical/physical reactions responsible for the wear rate. Severe wear is associated with metallic wear debris and unlike mild wear can be related to the mechanical properties of the materials involved. It has been shown f4] that with steel in a normal environment a transition from mild to severe wear occurs as subsurface deformation of the whole contact becomes plastic, i.e. the situation Suh applies to asperity interactions.
413
Clearly oxidation cannot play a part in the subsurface failure of the metal and the wear debris is thrown off as metallic plates. The approach adopted to predict the transition from mild to severe wear [4] was similar to that now proposed by Suh in that the contact stress situation was related to the mechanical properties of the metals. A real advance in the understanding of wear mechanics would be to distinguish the material properties determining the transition in wear behaviour as well as the wear rate. There seems to be a real opening here to develop the delamination theory in relation to the fundamental work on the cyclic stress/strain properties of metals that is currently being investigated in the area of fatigue mechanics. Conclusion In an inert atmosphere a cohesive deformed layer can form where the wear rate is determined by crack mechanics. However, in a normal environment the application of the delamination theory directly to wear appears to be limited. The oxidation and associated surface cracks result in a situation where the consideration of the mechanics of asperity deformation cannot readily be applied to predict wear rates. In a normal environment the delamination theory can be applied to a severe wear situation where metallic wear particles are produced. The study of cyclic stress/strain properties in this context in relation to wear behaviour offers interesting possibilities for the further development of the delamination theory.
T. M. BEAGLEY SKF Engineering
and Research
Centre B. V., Postbus
50, Nieuwegein
(The Netherlands)
1 N. P. Suh, An overview of the delamination theory of wear, Wear, 44 (1977) 1 - 16 and other papers. 2 J. F. Archard, Contact and rubbing of flat surfaces, J. Appl. Phys., 24 (1953) 981 - 993. 3 Y. P. Chiu, T. E. Tallian and J. I. McCool, An engineering model of spaling fatigue failure in rolling contact, Wear, 17 (1971) 433. 4 T. M. Beagley, Severe wear of rolling/sliding contacts, Wear, 36 (1975) 317.
(Received
August 26, 1977)
417 in the Netherlands
Letters to the Editor Comments on “The delamination theory of wear”
N. P. Suh [l] has identified a wear process that had not previously been observed and has vigorously pursued the phenomenon from a number of viewpoints. The large number of papers covering the subject will stimula~ a new awareness in what must be described as “wear mechanics” and will certainly lead to a higher level of understanding of the wear process. However, two overriding questions must present themselves to any researchers studying the plethora of papers on the delamination theory. Firstly, what is the real essence of the delamination theory, i.e. what are its bounds, and, secondly, how relevant is the theory to one’s own particular area of interest ? A general answer to the first question appears to be that the delamination theory is relevant to situations where the wear rate is determined by the mechanical properties of the materials concerned rather than by effects such as oxidation. This leads to the second question. The successful expe~ment~ evidence suppo~ng the theory has been obtained from wear experiments performed in inert atmospheres. The gener~ization of the theory to a normal situation where air and other surfactants are present has not been proved. It has already been well established that crack initiation can occur either through plastic failure of material at high subsurface stress levels or stress concentration caused by inclusions. The former is the basis of many wear theories [2] , whereas the latter has been studied extensively in connection with roller bearing fatigue failure [ 31. The question is whether, as postulated in the delamination theory, this process of crack initiation and propagation is related to the wear rate. Here the familiar catagorization of “mild” and “severe” wear should be considered. Mild wear encompasses the majority of wear processes where the wear debris has a high oxide content. It does not only apply to high speed sliding as implied by Suh. In mild wear the oxide formed on the surface of the metal prevents the formation of a homogeneous deformed layer and favours the formation of small surface cracks. Both effects result in a situation that is so dominated by surface oxidation processes that the delamination theory can no longer be applied. Lubrication adds to the complexity of the chemical/physical reactions responsible for the wear rate. Severe wear is associated with metallic wear debris and unlike mild wear can be related to the mechanical properties of the materials involved. It has been shown f4] that with steel in a normal environment a transition from mild to severe wear occurs as subsurface deformation of the whole contact becomes plastic, i.e. the situation Suh applies to asperity interactions.
413
Clearly oxidation cannot play a part in the subsurface failure of the metal and the wear debris is thrown off as metallic plates. The approach adopted to predict the transition from mild to severe wear [4] was similar to that now proposed by Suh in that the contact stress situation was related to the mechanical properties of the metals. A real advance in the understanding of wear mechanics would be to distinguish the material properties determining the transition in wear behaviour as well as the wear rate. There seems to be a real opening here to develop the delamination theory in relation to the fundamental work on the cyclic stress/strain properties of metals that is currently being investigated in the area of fatigue mechanics. Conclusion In an inert atmosphere a cohesive deformed layer can form where the wear rate is determined by crack mechanics. However, in a normal environment the application of the delamination theory directly to wear appears to be limited. The oxidation and associated surface cracks result in a situation where the consideration of the mechanics of asperity deformation cannot readily be applied to predict wear rates. In a normal environment the delamination theory can be applied to a severe wear situation where metallic wear particles are produced. The study of cyclic stress/strain properties in this context in relation to wear behaviour offers interesting possibilities for the further development of the delamination theory.
T. M. BEAGLEY SKF Engineering
and Research
Centre B. V., Postbus
50, Nieuwegein
(The Netherlands)
1 N. P. Suh, An overview of the delamination theory of wear, Wear, 44 (1977) 1 - 16 and other papers. 2 J. F. Archard, Contact and rubbing of flat surfaces, J. Appl. Phys., 24 (1953) 981 - 993. 3 Y. P. Chiu, T. E. Tallian and J. I. McCool, An engineering model of spaling fatigue failure in rolling contact, Wear, 17 (1971) 433. 4 T. M. Beagley, Severe wear of rolling/sliding contacts, Wear, 36 (1975) 317.
(Received
August 26, 1977)