Compacted oxide layer glaze
Encyclopedia
Compacted oxide layer glaze describes the often shiny, wear-protective layer of oxide
formed when two metals (or a metal and ceramic) are slid against each other at high temperature in an oxygen-containing atmosphere
. The layer forms on either or both the surfaces in contact and can protect against wear.
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 oxidizing 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 amorphous oxides of the same form as ceramic glazes, hence the name 'glaze' is still currently used.
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 disulfide
(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 mechanically 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
(also referred to as severe wear). With the 'glaze' layer present, such severe adhesive interactions cannot occur and wear may be greatly reduced. The continued generation of oxidized debris during the more gradual wear that results (entitled mild wear) can sustain the 'glaze' layer and maintain this low wear regime.
However, their potential application has been hampered as they have only successfully 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' - actually a brief period of adhesive or severe 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 the damage inflicted during the 'run in' period is one factor preventing this technique 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
.
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 / composition), 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 a loose, abrasive oxide to the formation of wear protective compacted oxide glaze layers and vice versa, or even the reappearance of adhesive or severe wear. Due to the complexities of the conditions controlling the types of wear observed, there have been a number of attempts to map types of wear with reference to sliding conditions in order to help better understand and predict them.
, where there is an increasing demand for ever higher efficiency and thus operating temperature
.
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
formed when two metals (or a metal and ceramic) are slid against each other at high temperature in an oxygen-containing atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
. 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 compacted oxideOxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
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 oxidizing 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 amorphous oxides of the same form as ceramic glazes, hence the name 'glaze' is still currently used.
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 disulfide
Molybdenum disulfide
Molybdenum disulfide is the inorganic compound with the formula MoS2. This black crystalline sulfide of molybdenum occurs as the mineral molybdenite. It is the principal ore from which molybdenum metal is extracted. The natural amorphous form is known as the rarer mineral jordisite. MoS2 is less...
(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 mechanically 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
Adhesion
Adhesion is any attraction process between dissimilar molecular species that can potentially bring them in close contact. By contrast, cohesion takes place between similar molecules....
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
Wear
In materials science, wear is erosion or sideways displacement of material from its "derivative" and original position on a solid surface performed by the action of another surface....
(also referred to as severe wear). With the 'glaze' layer present, such severe adhesive interactions cannot occur and wear may be greatly reduced. The continued generation of oxidized debris during the more gradual wear that results (entitled mild wear) can sustain the 'glaze' layer and maintain this low wear regime.
However, their potential application has been hampered as they have only successfully 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' - actually a brief period of adhesive or severe 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 the damage inflicted during the 'run in' period is one factor preventing this technique 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
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
.
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 / composition), 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 a loose, abrasive oxide to the formation of wear protective compacted oxide glaze layers and vice versa, or even the reappearance of adhesive or severe wear. Due to the complexities of the conditions controlling the types of wear observed, there have been a number of attempts to map types of wear with reference to sliding conditions in order to help better understand and predict them.
Potential uses
Due to the potential for wear protection at high temperatures beyond which conventional lubricants can be used, possible uses have been speculated in applications such as car engines, power generation and even aerospaceAerospace
Aerospace comprises the atmosphere of Earth and surrounding space. Typically the term is used to refer to the industry that researches, designs, manufactures, operates, and maintains vehicles moving through air and space...
, where there is an increasing demand for ever higher efficiency and thus operating temperature
Operating temperature
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
.