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Bronze aluminum composite coatings containing different amounts of alumina were fabricated by plasma spray process and their tribological properties were investigated using ball-on-disk (BOD) and rubber wheel (RW) tests at room temperature. Main wear mechanisms in pure bronze coatings during the ball-on-disk friction test were abrasion and intersplat delamination. The addition of alumina in bronze coatings clearly enhances their wear resistance. To explain this behavior, this article proposes an additional wear mechanism in the composite coatings that involves the rupture of the alumina lamellae located just below the wear track leading to a uniform distribution of fine alumina particles enveloped by the bronze matrix, which increase the surface hardness and hinder the wear. The deposition of debris on the wear track of composite coatings provokes an enhancement of the wear resistance as well. Bronze coatings show a low and stable friction coefficient of around l = 0.3. Nevertheless, coatings with reinforcing particles of alumina show an abrupt transition in the friction coefficient from values around l = 0.4-0.8, related to the modification of the surface contacts on the wear track due to the formation of a compacted debris layer deposited during the tribological test.

<div class="section abstract"><div class="htmlview paragraph">The oxidation of raw materials, such as phenolic resin, in the pad during the braking depends on the temperature but also on the oxygen diffusion capability through the pad. Determination of oxygen diffusion is a key point in knowing how deep from the surface tribochemistry can take place. The diffusion of oxygen through the pad is a drawback because it induces the matrix decomposition that contributes to intra-stop CoF instability and consequently worsens NVH. This study is focused on determining the oxygen diffusion through brake pads using oxidized iron sulphide particles as indicator parameter. Iron sulphide has a rough microstructure when it becomes oxide that can be recognized easily, making it a good marker. The oxygen diffusion was determined through the pad characterization, after SAE J2522 and J2707 tests, with SEM and EPMA/WDS techniques. The results show that tribochemistry also takes place beneath the PAD surface during braking due to the oxygen presence and the high temperatures reached. Depending on the testing schedule the temperature profiles on the pad are different and so is the tribochemistry. In pads tested according to SAE J2707, the oxidized particles are found in deeper sections than in pads tested according to SAE J2522. The results of this research will contribute to understanding where the decomposition of the resin starts, and therefore, how reactive compounds such as iron sulphide that also react with oxygen helps to keep the integrity of the matrix by decreasing the decomposition and contributing to the stability of intra-stop CoF, and so NVH properties.</div></div>

ResumenEn este artículo se utiliza la técnica de proyección fría (Cold Gas Spray -CGS-) para la obtención y caracterización de materiales compuestos constituidos por una matriz metálica de bronce de aluminio y una fase cerámica de alúmina con la intención de mejorar la resistencia al desgaste de los recubrimientos de bronce metálico puros. Se describen los diferentes procesos que ocurren durante la formación del recubrimiento (acritud de las partículas metálicas, rotura de las partículas cerámicas, efecto de granallado sobre el substrato metálico, entre otros) y se analizan cuáles son sus consecuencias sobre las propiedades del recubrimiento. Se han realizado ensayos de desgaste por fricción (ensayo Ball-on-Disk), abrasión (Rubber Wheel) y erosión, así como se ha determinado la microdureza y adherencia, y correlacionando los resultados obtenidos con el contenido de fase cerámica del recubrimiento. Se concluye que la fase cerámica reforzante incrementa sustancialmente las propiedades tribológicas con relación al bronce de partida. Finalmente se analizan los diferentes mecanismos de desgaste que tienen lugar durante los ensayos tribológicos. Palabras claveProyección fría; Unión estado sólido; Tribología; Compuesto; Bronce; Alúmina; Refuerzo; Desgaste. Metal-ceramic composite coatings obtained by new thermal spray technologies: Cold Gas Spray (CGS) and its wear resistance AbstractIn this paper, composite coatings composed by an aluminum bronze metal matrix and a hard ceramic alumina phase obtained by cold spray technique were obtained in order to increase the tribological properties of the pure bronze coatings. The different processes that occur during the coating formation (hardening of the metal particles, fragmentation of the ceramic particles, shot peening on the metal substrate, etc) are described and their effects on the coating properties are studied. Wear tests consisting on Ball-on-Disk tests, abrasion Rubber Wheel tests and erosion tests as well as microhardness and adhesion tests are carried out and the results are correlated with the ceramic phase content of the coatings. It can be concluded that the hard ceramic phase increases the tribological properties with relation of the initial bronze coating. Finally, main wear mechanisms during the tribological tests are described. KeywordsCold spray; Solid state joining; Tribology; Composite; Bronze; Alumina; Reinforcement; Wear. INTRODUCCIÓN Sobre el estado del arte de la proyección fríaEn los últimos años, las tecnologías de proyección térmica han evolucionado generando nuevos procedimientos y materiales que permiten realizar recubrimientos a medida para diferentes aplicaciones. Una de las limitaciones que existía en los procesos de proyección térmica convencionales (por ejemplo HVOF -High Velocity Oxy Fuel-, Plasma o llama) parece ser superado por un nuevo proceso denominado proyección fría (Cold Gas Spray, CGS). La proyección fría es un proceso de proyección en estado sólido, lo que hace posible la deposición de materiales susceptibles a la oxidación como ...

<div class="section abstract"><div class="htmlview paragraph">In order to keep the coefficient of friction stable, some additives such as metal sulphides, are included in the brake pads formulation. Previous work from RIMSA has shown that oxidation temperature range of the metal sulphides can be one of the key properties to explain their contribution to the performance and wear of a PAD. This new work is a step forward in the interpretation of the mechanism of sulphides as chemically active additives in the brake pads. Phenolic resin is the matrix of the brake pads and starts to decompose around 300 °C in presence of oxygen and temperature.</div><div class="htmlview paragraph">In order to establish a connection on between sulphide oxidation and phenolic resin degradation, several studies based on heat treatment of blends of different metal sulphides (Iron sulphide, Tin sulphide and Composite sulphide) with phenolic resin have been done. Then the material evolution was studied with techniques such as TGA-DSC, XRD, IR and SEM-EDS.</div><div class="htmlview paragraph">The results obtained confirm that there is an interaction between metal sulphides and resin, and the oxidation mechanism of both materials seems to be modified when they were blended. Studying PAD surface cross section after SAE J2522 and J2707, this work then introduces several SEM - EDS evidences that this mechanism can be found in the brake pads, and the results appoint that oxygen is present several tens of microns beneath the PAD surface during braking. This research will contribute to understanding the tribochemistry of the metal sulphides and phenolic resin that take place during braking and so the pad behaviour.</div></div>