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An often shiny, wear-protective layer of oxide formed when two metals (or a metal and ceramic) are slid against each other at high temperture in an oxygen-containing atmosphere. The layer forms on either or both the surfaces in contact and can protect against wear.
Background
A not often used definition of 'glaze' is the highly sintered compact oxide layer formed due to the sliding of either two metallic surfaces (or sometimes a metal surface and ceramic surface) at very high temperatures (normally several hundred degrees Celsius) in oxidising conditions. The sliding or tribological action generates oxide debris that can be compacted against one or both sliding surfaces and under the correct conditions of load, sliding speed and oxide chemistry as well as (high) temperature, sinter together to form a 'glaze' layer. The 'glaze' formed in such cases is actually a crystalline oxide, with a very small crystal or grain size having been shown to approach nano-scale levels. Such 'glaze' layers were originally thought to be amourphous oxides of the same form as ceramic glazes, hence the name 'glaze' is still currently used. Glaze is a thin shiny coating, or the act of applying the coating. ...
Sintering is a method for making objects from powder, increasing the adhesion between particles as they are heated. ...
An oxide is a chemical compound of oxygen with other chemical elements. ...
Such 'glazes' have attracted limited attention due to their ability to protect the metallic surfaces on which they may form, from wear under the high temperature conditions in which they are generated. This high temperature wear protection allows potential use at temperatures beyond the range of conventional hydrocarbon-based, silicone-based or even solid lubricants such as molybdenum disulphide (the latter useful up to about 450 °C short term). Once they form, little further damage occurs unless there is a dramatic change in sliding conditions.
Such 'glazes' work by providing a mechanicaly resistant layer, which prevents direct contact between the two sliding surfaces. For example, when two metals slide against each other, there can be a high degree of adhesion between the surfaces. The adhesion may be sufficient to result metallic transfer from one surface to the other (or removal and ejection of such material) - effectively adhesive wear. With the 'glaze' layer present, such severe adhesive interactions cannot occur and wear may be greatly reduced. The continued generation of oxidised debris during the more gradual wear that results can sustain the 'glaze' layer and maintain this low wear regime. Dew drops adhering to a spider web Water droplets adhering on a hibiscus petal Adhesion is the molecular attraction exerted between bodies in contact. ...
In materials science, wear is the erosion of material from a solid surface by the action of another solid. ...
However, their potential application has been hampered as they have only sucessfully been formed under the very sliding conditions where they are meant to offer protection. A limited amount of sliding damage (referred to as 'run in wear') needs to occur before the oxides are generated and such 'glaze' layers can form. Efforts at encouraging their early formation have met with very limited success and and the damage inflicted during the 'run in' period is one factor preventing this phenomena being used for practical applications.
As oxide generated is effectively the result of the tribochemical decay of one or both of the metallic (or ceramic) surfaces in contact, the study of compacted oxide layer glazes is sometimes referred to as part of the more general field of high temperature corrosion. Corrosion is deterioration of intrinsic properties in a material due to reactions with its environment. ...
The generation of oxides during high temperature sliding wear does not automatically lead to the production of a compacted oxide layer 'glaze'. Under certain conditions (potentially due to incorrect conditions of sliding speed, load, temperature or oxide chemistry / compositon), the oxide may not sinter together and instead the loose oxide debris may assist or enhance the removal of material by abrasive wear. A change in conditions may also see a switch from the formation of wear protective compacted oxide glaze layers to a loose, abrasive oxide and vice versa. In materials science, wear is the erosion of material from a solid surface by the action of another solid. ...
Potential uses Due to the potential for wear protection at high temperatures beyond which conventinal lubricants can be used, possible uses have been speculated in applications such as car engines, power generation and even aerospace, where there is an increasing demand for ever higher efficiency and thus operating temperature.
Compacted oxide layers at low temperature Compacted oxide layers can form due to sliding at low temperatures and offer some wear protection, however, in the absence of heat as a driving force (either due to frictional heating or higher ambient temperature), they cannot sinter together to form more protective 'glaze' layers.
References - I.A. Inman. Compacted Oxide Layer Formation under Conditions of Limited Debris Retention at the Wear Interface during High Temperature Sliding Wear of Superalloys, Ph.D. Thesis (2003), Northumbria University, ISBN 1581123213 (preview)
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