Coatings for Automotive Gray Cast Iron Brake Discs: A Review

Gray cast iron (GCI) is a popular automotive brake disc material by virtue of its high melting point as well as excellent heat storage and damping capability. GCI is also attractive because of its good castability and machinability, combined with its cost-effectiveness. Although several lightweight alloys have been explored as alternatives in an attempt to achieve weight reduction, their widespread use has been limited by low melting point and high inherent costs. Therefore, GCI is still the preferred material for brake discs due to its robust performance. However, poor corrosion resistance and excessive wear of brake disc material during service continue to be areas of concern, with the latter leading to brake emissions in the form of dust and particulate matter that have adverse effects on human health. With the exhaust emission norms becoming increasingly stringent, it is important to address the problem of brake disc wear without compromising the braking performance of the material. Surface treatment of GCI brake discs in the form of a suitable coating represents a promising solution to this problem. This paper reviews the different coating technologies and materials that have been traditionally used and examines the prospects of some emergent thermal spray technologies, along with the industrial implications of adopting them for brake disc applications

Microstructural Evolution and Sintering of Suspension Plasma-Sprayed Columnar Thermal Barrier Coatings

Suspension plasma spray (SPS) is capable of producing coatings with porous columnar structure, and it is also a much cheaper process compared to the conventionally used electron beam physical vapor deposition (EB-PVD). Although TBCs with a columnar microstructure that are fabricated using SPS have typically lower thermal conductivity than EB-PVD, they are used sparingly in the aerospace industry due to their lower fracture toughness and limited lifetime expectancy. Lifetime of TBCs is highly influenced by the topcoat microstructure. The objective of this work was to study the TBCs produced using axial SPS with different process parameters. Influence of the microstructure on lifetime of the coatings was of particular interest, and it was determined by thermal cyclic fatigue testing. The effect of sintering on microstructure of the coatings exposed to high temperatures was also investigated. Porosity measurements were taken using image analysis technique, and thermal conductivity of the coatings was determined by laser flash analysis. The results show that axial SPS is a promising method of producing TBCs having various microstructures with good lifetime. Changes in microstructure of topcoat due to sintering were seen evidently in porous coatings, whereas dense topcoats showed good resistance against sintering.First Online: 25 October 2018This article is an invited paper selected from presentations at the 2018 International Thermal Spray Conference, held May 7-10, 2018, in Orlando, Florida, USA, and has been expanded from the original presentation.</p

Influence of processing conditions on the microstructure and sliding wear of a promising Fe-based coating deposited by HVAF

Thermal spray is a versatile and cost-effective process to deposit wear and corrosion resistant coatings. In this work, a relatively new ‘Fe-based’ chemistry comprising boride and carbides, is explored as a ‘greener’ alternative to the relatively expensive and carcinogenic Co-based coatings to mitigate wear. The emergent thermal spray process of high-velocity air-fuel (HVAF) spraying was chosen to deposit the Fe-based coatings, with the high-velocity oxy-fuel (HVOF) also being employed solely for the purpose of preliminary comparison. Detailed characterization of the HVOF and HVAF sprayed Fe-based coatings was carried out. Microstructure, porosity, hardness and phase analysis results demonstrate the influence of processing conditions, where specific spray conditions yielded minimal undeformed particulates content, high hardness, low porosity and feedstock phase retention. Differences in microstructural features of the as-deposited coatings in relation to their processing conditions are discussed in detail. The coatings were subjected to ball-on-disc tribometry tests at different load conditions and their friction and wear performance were evaluated. The coefficient of friction results of investigated coatings concurred with their respective microstructural features. Post-mortem of the worn coating surface, the mating alumina ball surface and wear debris was performed using SEM/EDS analysis to understand the associated wear mechanisms and material transfer. This work provides new insights on identifying appropriate HVAF processing conditions to achieve acceptable microstructural features and phases in Fe-based coatings for improved wear performance