Heat treatment of plasma sprayed alumina-chromia composite coatings

Abstract

Atmospheric plasma spray technique is used to develop alumina-based coatings by spraying mechanically blended alumina-chromia feedstock with chromia content varying from 1-6 wt-%. The influence of annealing temperature (900-1200°C) on the phases formed in the coatings is also studied. Phases present in the coatings are analysed and quantified using Xray diffraction and Rietveld refinement, respectively. Microstructures of the coating cross sections are studied under scanning electron microscope. Wear rate of the annealed coatings are evaluated by linear reciprocating wear tests. The adhesion strength of the coatings is also estimated by ASTM C1624 standard. The results indicate that the increase in annealing temperature causes rise in transformation of metastable phases of alumina into stable αalumina. The minimum annealing temperature required to complete this transformation is estimated for each coating. The lowest specific wear rate is observed at 1150°C temperature and with 4 wt-% chromia content. The significant change in the trend of wear rate observed at the annealing temperature of 1200°C can be attributed to formation of large amount of cracks in the coatings. Adhesion strength of the coating improves with increasing chromia content. However, with increase in annealing temperature the coating adhesion decreases significantly due to the formation of cracks.

The Effect of Heat Treatment on the Microstructure and Phase Composition of Plasma Sprayed Al2O3 and Al2O3-TiO2 Coatings for Applications in Biomass Firing Plants

“…[21]. Whereas at temperatures below 570 • C and atmospheric pressure, only two phases are stable, magnetite (Fe 3 O 4 ) and hematite (Fe 2 O 3 ) [25]. However, as was reported by R. Y. Chen, at temperatures below 700 • C, oxidation results are inconsistent and significantly affected by sample preparation methods.…”

“…As can be seen, there are noticeably fewer changes in the phase composition of the Al2O3 coating after heating (Figure 11). The as-sprayed coating demonstrated the diffraction peaks at 2θ = 25.6° 35.7° 43.5° 57.7° and 63.1°, which were attributed to the rhombohedral α-Al2O3 phase [9,25,28]. The γ-Al2O3 phase was also determined at 2θ = 37.4° 38.9° 45.9° and 67.1° [22,25,28] The ratio of the highest intensity of α-Al2O3 (at 43.5°) and γ-Al2O3 (at 45.9°) peaks was 1.16 before heat treatment.…”

“…The as-sprayed coating demonstrated the diffraction peaks at 2θ = 25.6° 35.7° 43.5° 57.7° and 63.1°, which were attributed to the rhombohedral α-Al2O3 phase [9,25,28]. The γ-Al2O3 phase was also determined at 2θ = 37.4° 38.9° 45.9° and 67.1° [22,25,28] The ratio of the highest intensity of α-Al2O3 (at 43.5°) and γ-Al2O3 (at 45.9°) peaks was 1.16 before heat treatment. The initial phase in alumina feedstock powder was only α-Al2O3.…”

Effects of Laser-Remelting on the Microstructure, Hardness and Oscillating Wear Resistance of Atmospheric Plasma Sprayed Alumina-Rich Coatings

“…The presence of γ-Al 2 O 3 is generally assumed as the main reason that some alumina coating properties, such as corrosion resistance or electrical insulation, are noticeably worse than the properties of sintered alumina [1,15,16]. Retransformation of metastable γ-Al 2 O 3 to favored α-Al 2 O 3 can be reached through heat treatment or service temperature above 900 • C, with complete transformation occurring at 1200 • C [17][18][19]. Due to the density differences between γ-Al 2 O 3 and α-Al 2 O 3 as well as the large thermal stress caused by the high temperatures required, this phase retransformation is often related to extensive formation of defects in the coating, deterioration of the substrate properties or damage to the coating-substrate interface due to different thermal expansion coefficients [17,18,20].…”

Influence of Detonation-Spraying Parameters on the Phase Composition and Tribological Properties of Al2O3 Coatings

“…One of the promising technologies is thermal spraying. Gas-thermal methods made it possible to obtain high quality, durable coatings [1][2][3][4][5][6][7]. For example, detonation spraying (DS) and high velocity oxygen-fuel (HVOF) spraying provide excellent adhesion strength, low porosity and a tough, dense and wear-resistant coating [8][9][10].…”