50 establishments around the world which are capable of depositing EB PVD TBCs

50 establishments around the world which are capable of depositing EB-PVD TBCs to the required standard for engine service. Figure 5.3 shows an industrial-scale EB-PVD unit used for the deposition of coatings on turbine blading. It has been found that superior TBC density, hardness, erosion resistance and spallation life are achieved only when the substrate temperature is raised to within the range 850 ◦ C to 1050 ◦ C during processing. By design, the resulting morphology then consists of a series of columnar colonies which grow competitively in a direction perpendicular to the surface

286 Environmental degradation: the role of coatings Vertical rotary drive To vacuum pumps 45 kW/gun Vapour stream (thermal energy = 0.1 eV) Load lock chamber Part manipulator 0–14 rev/min, 2 rotary axis 0–1000 mm/min translation axis Chamber dimensions depth: 910 mm 896 mm 726 mm width: height: Maximum substrate dimensions/weight Horizontally fed cylinder: 200 mm dia. × 289 mm/20 kg Vertically held disc: 400 mm dia./100 kg Three ingot feeders 70 mm dia. × 500 mm long ingots 0.15 to 15 mm/min feed rate Four independent 8cm ion sources (100–1000 eV) 10 − 4 Torr Vacuum pumps Fig. 5.2. Schematic illustration of the arrangement used for the electron beam physical vapour deposition (EB-PVD) method [15]. of the substrate. Figure 5.4 shows the microstructure of a typical 7 wt%Y 2 O 3 –ZrO 2 thermal barrier coating produced by the EB-PVD process, which has been shown to grow with a strong 111 texture [16]. The boundaries between the columnar deposits have been shown to be poorly bonded [17] – in effect, a bundle of columnar grains exists whose ends are attached to the substrate – so that a degree of strain tolerances is introduced into the coatings which would otherwise be prone to extreme brittleness and failure due to stresses developed during thermal cycling. Unfortunately, this also means that the coating quality is impaired if the surface to be coated is not perfectly clean; small imperfections are not covered up as they might be with other coating processes, but can result in a growth abnormality that is magnified as the coating thickens [7]. Experiments have shown that the size and morphology depends not just upon the substrate temperature, but also on the rate of component rotation which is required due to the complicated shape of the turbine aerofoils – EB-PVD being a line-of-sight process. At low temperatures and low rotational speeds, the columns vary not only in size from root to top, but also from one to another; see Figure 5.5. Increasing both the substrate temperature and the rotational speed improves the regularity and the parallelity of the microstructure and enlarges the column diameter. The need for component temperatures to be raised in this way prior to coating places further demands on the coating apparatus. For this purpose, either conventional radiation heating in the form of graphite furnaces placed in a preheating chamber, or else pre-heating by the electron beam itself is used.