Assessment of corrosion-induced damage in the mechanical contact response of cemented carbides at different length scales
- Autores: Yefeng Zheng
- Directores de la Tesis: Luis Llanes Pitarch (dir. tes.), Gemma Fargas Ribas (codir. tes.)
- Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2020
- Idioma: español
- Materias:
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- Resumen
Cemented carbides, also referred to as hardmetals, are composite materials consisting of brittle refractory carbide phase (generally WC) embedded in a metallic matrix. Such microstructure leads to an outstanding combination of hardness, wear resistance and toughness. This makes hardmetals first materials in several highly demanding applications, e.g. cutting or forming of metallic alloys, as well as mining operations.
Several of the above applications imply exposure to chemically aggressive media, such as lubricants, petrochemical and mine slurries, seawater, etc. Under these conditions, it has been shown that failure induced under applied load is accelerated, and corresponding service life may be significantly shortened. In this regard, the detrimental corrosion-related effects on tribological response and effective wear resistance of cemented carbides have aroused the greatest concern. However, investigations addressing similar information linking corrosion-induced damage and contact mechanical response at different length scales are quite limited.
Within the above framework, the first part of this thesis was devoted to carry out a systematic and comprehensive study about corrosion-induced damage and residual strength (damage tolerance) for four microstructurally different hardmetals exposed to various corrosive media. It is found that acidic medium led to higher corrosion rates and more significant strength degradation than those from neutral and basic ones. Regarding degradation mechanisms, it is evidenced that corrosion starts at binder pool centers and evolves towards binder/WC interfaces when exposed to acid solution, while in the basic one it is initially located at binder/WC interfaces and subsequently expands into the ceramic particles.
The subsequent sections were focused on assessing the corrosion-induced changes on the mechanical contact response of hardmetals through an increasing length scale (from 100s nanometers to 1000s microns). First, nanoindentation and nanoscrach techniques were employed to assess the influence of corrosion on the mechanical integrity of hardmetals. Corrosion-induced changes on corresponding response and damage scenario are discussed. It is concluded that dissolution of metallic phase becomes critical as it leads to an effectively lessened mechanical integrity.
A similar investigation was then extended to a higher length scale range (from 10s to 100s of microns in depth), combining relatively long corrosion times with pyramidal indentation and sliding contact (microscratch) tests, in order to evaluate corrosion-induced changes on both load-bearing capability and damage scenario of hardmetals. Results reveal that mechanical contact strength and resistance to crack extension are significantly reduced after exposure to corrosive media. Such lessening effects are found to depend on the ratio between indentation and/or scratch depth and thickness of the corroded layer. Alike pronounced corrosion influence is evidenced in surface and subsurface damage scenarios regarding the crack propagation behavior.
Finally, an even higher length scale (up to 1000 of microns) was introduced by combining Hertzian indentation techniques and variable corrosion times. Corrosion effects on corresponding mechanical response and damage were assessed for hardmetals with metallic binders of different chemical nature. Results indicate significant corrosion-induced changes on indentation stress-strain response and damage scenario. In this regard, critical loads for emergence and evolution of specific damage events - ring and radial cracks, and even specimen failure - are proposed as figures of merit for material selection under the combined action of corrosion and contact loading. It points out the consideration of the synergic interaction between corrosion resistance and hardness/toughness correlation for microstructural design optimization of hardmetals under service-like conditions.
