The application of coatings to protect parts and components against various types of wear mechanisms is notorious in several industrial sectors. The coatings can be applied using different processes and alloys, being, without a doubt, an efficient alternative to provide an increase in the component's lifespan. However, this application favors the emergence of several operational and metallurgical challenges.
At first, it is important to highlight that each specific situation and application requires a different solution.The conditions of the tribological systems (component, counter part, interfacial element and operating environment) dictate the necessary characteristics and properties that the component subject to wear must contain in order to protect it and increase its life span. In addition, the active wear mechanism (abrasive, adhesive, corrosive, erosive, cavitation) or a combination of these, is also a mandatory factor.
Regarding the wear resistance indicators, the mechanism involved is a primary system. For example, for abrasion stresses, the mechanical property of hardness is generally a satisfactory indicator. In several studies involving tribological analyzes of different coatings on different tribological system conditions, results are found that the hardness behaved inversely proportional to the loss of mass, the volumetric loss or the wear rate. However, contrary results are also reported by some authors. In this way, hardness is not a predictive characteristic, but only an indication of abrasion resistance. A similar description is also found regarding the friction coefficient.T he same can be observed regarding the toughness, elastic modulus, among several other mechanical properties.
Regarding hardness, we have for example the relationship proposed by Khruschov, which relates the hardness of the abrasive with the hardness of the material through a direct division, providing a value that can be consulted in a table, from which the regime of wear can be considered moderate, transitional, or severe. However, the model is applicable only under specific conditions. Hardness can also be a crucial property on the transition from wear mechanisms to abrasion conditions.
Another key element in the wear resistance of a coating is its microstructure. Aspects related to the matrix, such as hardness, ductility and tenacity; the phases present, such as size, geometry, concentration in the matrix and its mechanical properties; and the hardening mechanisms can have a direct influence on tribological behavior. In the case of coatings, the microstructure is established in function of several factors, which range from the materials, conditions of the coating deposition process, and application of post-coating treatments. Regarding abrasion, the dilution (understand how the dilution is calculated in the article) is widely indicated in the literature as being one of the most influential factors on the characteristics of microstructure, hardness and wear resistance.
The environment conditions where the component operates must also be observed. A high wear resistance coating in ambient temperature conditions can perform poorly at high working temperatures. Humid environments, with the presence of suspended particles, and chemical atmospheres require components with specific characteristics. Besides that, extreme environmental conditions can trigger the combined action of degassing mechanisms.
This topic is extremely interesting and complex to be analyzed, making it difficult to infer certain conclusions. In fact, the most prudent alternative, especially when it comes to assessing the tribological performance of coatings, is still experimental research.
We thank Msc. Eng. Jurandir Marcos Sousa for the valuable technical contribution in co-authoring the article.
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