Development and experimental research of a vacuum infiltration sintering apparatus

Wear-resistant seal coating on the tip of single-crystal turbine blades plays a crucial role in ensuring the airtightness and operational efficiency of aeroengines. With the continuous increase in turbine inlet temperature, the harsh service environment imposes increasingly stringent requirements on the coating, such as superior high-temperature wear resistance, oxidation resistance and adhesion. However, existing preparation technologies face prominent challenges: thermal spraying and laser cladding produce coatings with flat surfaces where abrasive particles are uniformly distributed inside, failing to meet the protruding morphological requirement; electrodeposited yields coatings that suffer from insufficient adhesion and increased brittleness with increasing thickness; brazing, while improving interface bonding, damages the base metal because of the diffusion of melting-point-lowering elements. Additionally, Al₂O₃ ceramic particles, as ideal reinforcing phases, exhibit poor wettability with metal melts, and their particle size and content significantly affect coating quality, yet relevant systematic research is scarce. To address these issues, this study aims to develop a high-performance preparation technology for NiCoCrAlYTa–Al₂O₃ blade-tip wear-resistant coatings, synergistically integrating the high-temperature wear resistance of particles and protective performance of the coating while avoiding damage to the single-crystal base metal.

A visualized high-vacuum infiltration sintering apparatus was developed based on a traditional vacuum tube furnace. Key improvements included equipping a 10× visual window, a high-speed camera (maximum shooting rate of 3980 frames/s), and a synchronous light source for real-time recording of the entire experimental process, as well as integrating a high-flux diffusion pump, vacuum gauge, and vacuum meter to achieve a high-vacuum environment of 1 × 10^–3 Pa with real-time monitoring. Al₂O₃ particles with three sizes (1, 10, and 30 μm) and five weight percentages (0%, 3%, 5%, 8%, and 10%) were selected as reinforcing phases, NiCoCrAlYTa as the coating matrix, and NiCrSi as the infiltration alloy. Electroless Ni–P alloy plating was applied to Al₂O₃ particles to improve their wettability with the metal melt. The coating preparation followed a specific thermal cycle: heating to 420 ℃ at 10 ℃/min for 30 min, subsequent heating to 1200 ℃ at 10 ℃/min for 2 h, cooling to 600 ℃ at a rate not exceeding 5 ℃/min, and final natural cooling to room temperature. The microstructure and surface morphology of the coatings were characterized using precision image measuring instruments, scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), and 3D profilometers. Friction and wear tests were conducted on a self-designed rig with a normal load of 1.5 N, sliding speed of 1 m/s, and total sliding distance of 1000 m; the wear resistance was evaluated by measuring the weight loss of coatings and mating graphite disks.

The electroless Ni–P plating effectively improved the wettability of Al₂O₃ particles with the metal melt, reducing the contact angle from 93.425° to 80.371°. Particle size had a considerable impact on coating formation: coatings reinforced with 1 and 10 μm Al₂O₃ particles contained numerous pores due to particle agglomeration, while those with 30 μm Al₂O₃ particles did not exhibit pore defects, resulting in dense coatings with protruding Al₂O₃ particles exhibiting an exposure height of 220–240 μm, which met the morphological requirements. Regarding mass percentage, when the Al₂O₃ content was ≤5%, the total coating porosity remained stable at approximately 0.6% with gas pores as the main defects; beyond 5%, particle agglomeration intensified, clogging seepage channels and leading to a sharp increase in porosity (0.56% for the 3% sample and 2.22% for the 10% sample). Friction and wear test results showed that all coatings containing Al₂O₃ particles exhibited considerably lower wear rates than the coating without Al₂O₃. The sample with 8% 30 μm Al₂O₃ particles achieved the lowest wear rate of 0.00151 mg·N^–1·m^–1, and the mating graphite disks formed effective wear marks with a depth of 60–90 μm, indicating excellent wear resistance and cutting–sealing performance. The main wear mechanism of the coatings was the spallation of protruding Al₂O₃ particles, and the wear rate increased at 10% Al₂O₃ content because of excessive particle agglomeration.

This study successfully developed a visualized high-vacuum infiltration sintering apparatus that enables real-time monitoring of the sintering process. The optimal preparation parameters were determined as follows: 30 μm Al₂O₃ particles with a mass percentage of 5%–8%, sintering temperature of 1200 ℃, and holding time of 2 h. The NiCoCrAlYTa–Al₂O₃ coating prepared under these parameters exhibits excellent comprehensive performance, including low porosity, high wear resistance, and strong adhesion without damaging the single-crystal base metal. This technology solves the key technical problems of existing preparation methods and provides important theoretical and experimental support for the engineering application of high-performance wear-resistant seal coatings on single-crystal turbine blade tips.