Method and Apparatus for Depositing Stable Crystalline Phase Coatings of High Temperature Ceramics

Producing high temperature coatings on ceramics using a high enthalpy air plasma spray torch and supplying in at least 3 passes to deposit the coating is advantageous for spraying high temperature ceramics while avoiding formation of undesirable phases, if the stand off distance is chosen such that a width of a bead produced from a single spray pass on the substrate at ˜800° C. is less than 70% of a diameter of a plume of the torch at the stand off, and neither assisted heating, nor forced cooling, nor subsequent heat treatment is used. The rapid cooling endemic to thermal spray that leads to amorphous, metastable and other undesirable phases of alkaline earth aluminosilicate (e.g., barium-strontium aluminosilicate (BSAS)), rare earth silicates (RESs), mullite, etc. can be mitigated sufficiently by the close stand off and high enthalpy torch to provide highly crystalline and stable phase coatings.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention was discovered with experiments using a high enthalpy APS torch. Given the problems reported in the prior art with cracking of substrates, it is surprising that using a much higher enthalpy torch, and a much shorter stand off distance, no substrate cracking is observed, and given the absence of any reproducible report of thermal sprayed coatings deposited without assisted heating, the very high purity observed was remarkable.

FIGS. 1a,bschematically illustrate a high enthalpy air plasma spray apparatus respectively showing a stand-off distance typically used for thermal spraying in accordance with the prior art, and an embodiment of the invention.FIG. 1a, because of the larger stand off, illustrates less of the apparatus, however similar supply equipment is required for a high enthalpy torch10, in either case, with the proviso that prior art systems typically do not spray the same ceramics that are an integral part of the spray system inFIG. 1b.As shown inFIG. 1b,the apparatus includes a hopper13connected to the torch by a feed supply12. The hopper13contains a high temperature ceramic powder14, composed of a ceramic that melts above ˜1600° C., that is delivered to the torch10by the feed supply12in a manner known in the art. The ceramic used in accordance with the present invention tends to form amorphous, metastable, or otherwise undesirable phases if rapidly cooled below a temperature of about 900° C., and the invention mitigates the formation of the undesirable phases. For example, the ceramic may be an alkaline earth aluminosilicate (e.g., BSAS), RES, mullite, or an alumina or titania (doped or pure). A plasma gas supply 15 (partially shown) is coupled to the torch10to supply plasma gas. Preferably the plasma gas comprises a mix of plasma gasses that together provide 1) sufficient thermal conductivity to the ceramic particles, 2) sufficient enthalpy to heat the ceramic particles to a desired temperature, for example, above 2000° C., and 3) a low enough threshold for ionization to readily initiate plasma formation and low enough enthalpy to avoid melting the electrodes of the torch10. Conveniently 1)-3) may be ensured by providing: sufficient concentrations of H2(e.g., 5-40%, more preferably 10-25%), N2(e.g., 30-80%, more preferably 45-50%), and Ar plasma gas (e.g., 10-80%, more preferably 25-45%), respectively, although other plasma gasses (e.g., He) could be added or substituted to some degree. The torch10includes an electrical power supply19(in partial view) for ionizing the gas to form the plasma, which defines a plume16, which is typically relatively stable in shape and size during constant supply conditions. The particles of the ceramic powder14, entrained in the plume16, melt, at least at their surface, and are accelerated by the plume16, to be ejected as a spray jet17. The parameters of the spray jet17(velocity distribution, diameter, temperature distribution, size distribution, etc.) are not as uniform as the envelope of the plume16, which can typically be maintained in a steady state, and keep a constant shape.

The spray jet17is rapidly cooling, and gradually decelerating in flight, until it strikes a coating surface of a substrate18. Substrate18is formed of a ceramic, such as a Si-based ceramic matrix composite (including SiC, and Si3N4). In accordance with the present invention, the substrate18is positioned a relatively short stand off distance D from the torch10. This obviates the need for assisted heating. The stand off distance D is chosen so that the diameter of the plume16at D is appreciably larger than the main core of spray jet17, which forms the individual spray beads. The substrate18is typically supported by a sample holder18in a manner that permits the spray jet17to scan across the coating surface, either by motion of the torch10or of the sample holder18(i.e., the motion normal to the direction of spraying, such as shown by arrow11).

For a stand off distance D shorter than the plume length16, the shorter the stand off, the less dwell time the particles have within the plume16, and accordingly the less energy is absorbed and the slower and colder spray jet17particles are, but the heat transfer from the plasma plume to the substrate is maximized. Conversely, the greater the stand off (until a distance beyond the plume's length), the greater the temperature and speed imparted to the spray jet17(up to a critical point), and also a wider and less focused spray jet17is at contact with the surface. The greater the stand off, the lower the heat transfer from the plasma plume to the substrate. The less focused the spray jet17, results in a greater variability of the deposition conditions (particle angle, local surface temperature, particle temperature and velocity, etc.), leading to a less definite and compact deposition trace (continuous sequence of deposition produced by the scanning) known as a bead. Applicant postulates that if the stand-off distance D is chosen so that a width of a single bead on a substrate at ˜800° C. is less than 70% of a diameter of the plume16of the torch10at the same stand off distance, and the coating is produced with at least three passes of the spray jet17, substantially less of the undesired phase is produced, as sufficient substrate heating and dwell time of the particles are provided.

The bead width is a sensitive parameter, and may vary widely with changes in the spray jet conditions and deposition conditions. The stand off where the bead has a certain width may reliably be determined by providing stable feed conditions, and spraying the substrate while holding a temperature of the substrate constant, such as at 800° C. This not to be understood as a requirement for the substrate to be at 800° C. during coating deposition, but merely as one possible manner of reliably determining a bead width as a function of stand off. Coating deposition in accordance with the present invention is provided without assisted heating or cooling, and without post spray heat treatment or annealing.

In sum, it is believed that by providing a thin bead in a wide plume, the heat required for gradual enough cooling to provide favourable nucleation kinetics (e.g., sufficient duration at or above 1000° C.) is provided, to avoid deposition of undesirable phases. In all aspects of the invention, however, Applicant does not wish to be limited by the foregoing theory.

Alternatively viewed, the invention comprises operating the high enthalpy air plasma spray torch10with the ceramic powder14to produce a coating on substrate18using a set of deposition conditions that substantially conform with the following norms: |[N2]|=47.5±2.5%, |[H2]|=17.5±7.5%, |[Ar]|=35±10%, |TGF|=280±30 slpm, |P|=110±23 kW, |D|=8±2 cm where [N2], [H2], and [Ar] are respectively the percentages of N2, H2, and Ar in the plasma plume, TGF is the total gas flow, P is the torch power, and D is the stand off distance measured in cm. Strict adherence to these limits are not believed to be required, as taking a slightly higher N2or H2fraction or lower Ar fraction may be mitigated by higher coolant rates, and/or slightly lower total gas flow rates, for example. Accordingly, a cumulative deviation from these norms of less than 20%, more preferably of less than 10%, and more preferably of less than 5%, or 1% are likely to be operable and provide the advantages of the present invention, subject to the limits on safe use of the high enthalpy air plasma spray torch, and an acceptable deposition efficiency.

Although not necessarily mandatory, pre-heating of the surface of the substrate using the torch can be employed. The typical pre-heating temperatures are above ˜500° C., more preferably ˜800-1000° C.

The same mullite powder (SG #1020, d10=15 μm/d50=30 μm/d90=50 μm, Saint-Gobain, Worchester, Mass., USA) was sprayed using a conventional 40 kW APS torch (F4-MB, Sulzer Metco, Westbury, N.Y., USA), and a high-enthalpy 120 kW APS torch (Axial III, Northwest Mettech, North Vancouver, BC, Canada). In the case of conventional spraying, a cooling system consisted of air lets was used to cool the front size of the substrate to limit the maximum coating surface temperature reached during deposition to ˜100° C. High-enthalpy spraying was performed without assisted heating or cooling, and the maximum coating surface temperature during deposition reached values of ˜1000° C.

A carrier gas (Ar at 9 slpm) was used to feed the mullite powder in a conventional manner, delivering the powder at a rate of approximately 4 g/min. Specifically, the high enthalpy torch spray parameters were as follows: torch plume gasses were: N245%, Ar 45%, and H210%, total gas flow 280 slpm, power ˜120 kW, and a stand off distance was 6 cm. The diameter of the torch nozzle was 0.5″ (1.27 cm). The plume had a length greater than 6 cm. The substrate was SiC Hexaloy SA™ (Saint-Gobain, Niagara Falls, N.Y., USA). Pre-heating of the substrate with the torch was not employed in this specific case.

Both coatings were analyzed via XRD, and the results are shown inFIG. 2. The XRD technique was employed as an attempt to compare the degree of crystallinity of the coatings. The ratio of peak areas between 20° and 40° to the total integrated area in this region was measured and defined as a crystallinity index (CI). The CI value is proportional to the degree of crystallinity of the coating, at least on its surface. The high-enthalpy spraying produced highly crystalline mullite coatings (B: crystallinity index CI≧0.95). These highly crystalline coatings also did not exhibit signs of debonding or delamination when deposited on SiC-based substrates. In comparison, the CI of the conventional sprayed mullite was about 0.25, and the XRD spectrum shows the typical amorphous hump across the 20-40° range (A).

Because X-rays used in regular XRD analysis penetrate the sample to a depth of few microns below its surface, and as the coating temperature was raised during deposition from room temperature, it might be hypothesized that some amorphous residual mullite phase may have been formed in the initial few layers (passes) of coating deposition. This residual amorphous mullite, if formed, is unlikely to be detected via regular XRD probing. In order to investigate whether amorphous mullite was present in the microstructure of the as-sprayed coatings, they were heat-treated at temperatures above the known crystallization point of mullite (i.e., ˜1000° C.) and examined using differential thermal analysis (DTA). The temperature was raised at a rate of 10° C./min up to 1200° C. in an N2atmosphere. The results of DTA are shown inFIG. 3. A shows a strong peak of the conventional APS sprayed coating, whereas B shows no peak at 989° for the high enthalpy APS sprayed coating.

In order to perform a valid comparison, the mass of each coating evaluated via DTA was similar (±30 mg). The DTA exhibited an exothermic phase transformation peak at 989° C. for the low crystallinity coating (CI=0.25), very similar to that observed by Lee et al. [12] for a highly amorphous mullite thermal spray coating, which corresponds to the crystallization temperature of the mullite. The coating deposited with the high-enthalpy APS torch, did not exhibit any peaks at this region, confirming the mullite coating contained no amorphous phase through its overall microstructure.

This powder was fed to a high enthalpy APS torch. The coating was applied overtop of an interlayer of mullite (deposited as described above). The powder feed was assisted by a carrier gas (Ar at 9 slpm) at a powder feed rate of ˜4 g/min.

Table 1 lists spray parameters used to produce BSAS coatings and mullite coatings with the Axial III APS torch (torch nozzle diameter 0.5″ (1.27 cm)). The first three rows are better suited to BSAS deposition, and the last two sets are better suited to mullite deposition. Specifically, the density of BSAS coating produced with the deposition conditions according to bottom two rows were found to be too dense, precluding the easy formation of vertical cracks, and the deposition conditions according to the first 3 rows produce mullite with lower density than is desired. Many other thermal spray conditions could be varied that would have an expected effect on the thermal spray conditions resulting in similar coatings, as is well known in the art, and thus features such as feedstock characteristics (e.g. morphology and porosity), feedstock particle size distribution, and feedstock powder feed rate, inter alia can have some impact on the optimal settings.

The BSAS powder was sprayed using a high-enthalpy APS torch (Axial III, Northwest Mettech, North Vancouver, BC, Canada). The coating deposition occurs via a scan pattern of the APS torch on the substrate surface, which sprays successive layers. The temperature of the surface of the coating was monitored with a pyrometer, which is placed behind the APS torch. It is important to point out that the pyrometer measures the temperature aimed over a specific point (target) on the coating surface, i.e., the overall surface coating temperate and the substrate temperature are not measured.

The maximum measured coating surface temperature at the target of the pyrometer was at least ˜1000° C. when the high-enthalpy torch was operated at ˜113-118 kW, and at least 800° C. when operated at ˜87 kW both at a stand off distance of 6 cm. The coating surface temperature is challenging to measure, due to the fact that this event occurs when the high energetic plasma plume and spray jet is over the specific pyrometer target. Thus there is a spatial and/or temporal lag between when the surface is maximally heated and when the temperature is measured. Therefore, the real maximum coating (or substrate) surface temperature is higher than measured via a pyrometer or infra-red camera.

FIG. 5is a SEM image showing an as-sprayed barrier system on the SiC substrate, the barrier system having the Si bond coat, the mullite interlayer and a BSAS top coat deposited on a SiC Hexoloy SA™ substrate. The microstructure inFIG. 5shows that the Si bond coat appears as a dense and crack-free layer showing good adhesion to the substrate (i.e., no gaps or spallation). Likewise good adhesion is shown between the interfaces of the Si bond coat, the mullite interlayer and the near-crack free BSAS top coat. This appears to be the first report of an as-sprayed crystalline BSAS coating (Celsian phase) produced without assisted heating or post spray annealing, and the first report of an as-sprayed crystalline mullite coating produced without assisted heating or cooling or post spray annealing.

As the standard attempt to simulate the lean environment of a gas turbine [2], the barrier system was subjected to a thermal treatment at 1300° C. (1 atm) in a continuous flow (˜3.5 cm/s) of water vapour (90% H2O, 10% air) inside a tube furnace for up to 500 h.FIG. 6includes an image of the same barrier system, showing the effects of heat treatment at 1300° C. for 500 h in a water vapour environment (left). An enlargement of the SiC substrate/bond coat interface (bottom right) shows no appreciable difference before and after the heat treatment. An image of an unprotected SiC Hexoloy SA™ substrate after the same type of heat treatment is shown at the top right. After 1300° C./500 h in water vapour, a SiO2scale loosely adhered to the substrate was formed over the SiC surface. Severe through-cracks were formed throughout this silica layer. Therefore, the mullite and BSAS coatings deposited in accordance with an embodiment of the present invention effectively protected the Si-based ceramics substrate against oxidation and water vapour attack.

The BSAS coating composition was analyzed in several manners.FIG. 7shows X-ray diffraction (XRD) spectra of BSAS powder (Amperit 870.084, H. C. Starck, Newton, Mass., USA), the as-sprayed coating, and the coating after exposure to a high temperature, aggressive heat treatment. Both which substantially consist of the Celsian monoclinic phase (Ba0.75Sr0.25Al2Si2O8), as identified based on the JCPDF #38-1451. The as-sprayed coating basically exhibits the same crystalline structure, i.e., the diffraction peaks overlap those of the powder. No evident amorphous hump is observed. Therefore, this as sprayed coating appears to be highly crystalline, exhibiting predominantly the celsian phase. After heat treatment at 1300° C. for 500 h, substantially the same XRD pattern is produced.

Harder and Faber [8] studied the amorphous/Hexacelsian-to-Celsian phase transformation kinetics of BSAS. They subjected samples to heat treatments between 1100 and 1400° C. from 30 min to 40 h. At 1100° C., there was no discernable transformation up to 40 h of heat treatment. Phase transformation to Celsian began to be observed at temperatures of 1200° C. This explains why the prior art typically requires annealing at temperatures above 1250° C.

The exclusivity of the Celsian phase in the as-sprayed coating can be explained by positing that the real maximum coating surface temperature at the plasma jet spot was higher than 1200° C. By using a spray jet diameter that is smaller than the plume diameter at the surface, the region under the spot is subjected to an intense heat provided by the plume, providing for annealing in real time of the coating during its deposition.

FIG. 8is a scanning electron micrograph showing in plan view, a single bead of BSAS sprayed at ˜120 kW (SiC Hexoloy SA™ substrate surface pre-heated with the torch at ˜800° C.). Arrows point to inclusions that were partially molten in deposition. About half of the surface of the bead is covered by such semi-molten regions. This considerable amount of semi-molten particles will likely influence the energy necessary to nucleate Celsian from amorphous and metastable Hexacelsian (Harder and Faber [8]) during the concurrent annealing and deposition performed by the high enthalpy APS torch.

FIG. 9is a XRD spectrum of the single BSAS bead ofFIG. 8and the XRD spectrum of a BSAS coating (˜180 μm thick/SiC substrate surface pre-heated with the torch at ˜700° C.) sprayed at ˜85 kW (maximum coating temperature ˜800° C.), using the same APS torch, plasma gas composition and spray distance (6 cm). The XRD spectrum shows that in spite being deposited at similar surface temperature conditions, the bead exhibits a significant amount of amorphous phase, characterized by the amorphous hump between the angles of 20° and 40°. On the contrary, the coating exhibits a highly crystalline Celsian phase. Based on this information, it is advisable to deposit the BSAS in at least 3 and preferably in at least 5 passes to perform annealing in real time. The conditions employed in high-enthalpy APS torches, such as, higher torch power and total gas flow than those of conventional ones, help to explain the fact that other inventions had to employ holding time and/or post-spray annealing to produce BSAS coatings exhibiting at least 50 vol % of celsian phase. The higher power levels and total gas flows, in addition to the high enthalpy N2gas, provided by the high enthalpy APS torch create the conditions for annealing the sample in real time during spraying, thereby inducing the formation of the celsian phase in as-sprayed coatings, without the necessity of a holding time and/or post-spray annealing. This annealing in real time is carried out throughout the successive deposit of spray beads, layers and torch passes over the substrate surface, because one single pass forming a single bead does not promote the formation of a highly crystalline celsian phase.

Harder and Faber [8] studied the transformation kinetics of BSAS coatings produced via APS from a fully crystalline celsian powder. As-sprayed coatings exhibited a mixture of amorphous, metastable Hexacelsian and Celsian BSAS phases. The presence of the Celsian phase was attributed to the presence of semi-molten particles embedded in the coating microstructure, which was estimated to be ˜10 wt %. By employing electron backscattered diffraction (EBSD) it was observed that the transformations from amorphous/metastable Hexacelsian phase to the Celsian phase at high temperatures were strongly influenced by the Celsian phase. The semi-molten BSAS celsian particles embedded in the as-sprayed coating acted as seeds for the nucleation of the Celsian phase transformation from the amorphous and metastable hexacelsian, upon annealing. Harder and Faber concluded that the energy necessary to transform APS BSAS coatings will be heavily influenced by the semi-molten particles [8], i.e., the higher the amount of semi-molten BSAS Celsian particles embedded in the coating microstructure, the lower the amount of energy required to nucleate the amorphous/Hexacelsian BSAS into stable crystalline Celsian.

Based on the equations determined by Harder and Faber [8], it is possible to calculate that it is necessary to reach a temperature of 1850° C. for ˜8 seconds to induce the phase transformation of 50% of Celsian phase from metastable Hexacelsian. Over a 4 cm×4 cm substrate area, the ˜2.5 cm diameter plasma plume spot stays ˜8 seconds over the surface of the coating during one complete set of torch passes (single beads) to produce a single layer.

In order to obtain an approximate effect of the substrate surface heating by the spray torch, the following experiment was carried out. The high enthalpy APS torch was set up at the same conditions to deposit a BSAS coating and placed standing still in front of a SiC-based CMC substrate for 5 seconds, but no BSAS powder was fed. After this interval, the torch was set aside in a rapid thrust to open a line of sight for the infra-red (IR) camera to immediately measure the substrate surface temperature. The temperature profile can be seen inFIG. 10.

The IR camera used can measure temperatures from 200° C. to 1600° C. When the torch is shifted off the substrate surface, and the IR camera comes in line of sight with the substrate surface, the measured temperature increases abruptly to values higher than 1600° C. (i.e., the limit of the equipment). The temperature is not registered for ˜1.5 s, until it cools down to 1600° C. and below. Therefore, during a continuous deposition when multiple torch scans are employed to build the coating on the substrate surface, using the conditions described in this invention, the coating surface temperature levels under the plasma plume spot likely reach values higher than 1600° C. When the BSAS coating surface is exposed to these temperature levels for some seconds, the thermodynamic conditions required to induce the amorphous/Hexacelsian phase transformation to stable crystalline Celsian seem to occur.

REFERENCES

Other advantages that are inherent to the structure are obvious to one skilled in the art. The embodiments are described herein illustratively and are not meant to limit the scope of the invention as claimed. Variations of the foregoing embodiments will be evident to a person of ordinary skill and are intended by the inventor to be encompassed by the following claims.