Ultrathin laminated oxide coatings

Abstract: A wear resistant article (10), such as a cutting tool. A hard ceramic or cemented carbide substrate (12) is coated with a laminated oxide coating (16) about 0.3-20 microns thick, having at least three ultrathin layers (18, 20, 22, 24, 26). Each layer (18, 20, 22, 24, or 26) is an oxide layer about 0.1-3 microns thick, predominantly of a different material than adjacent layers. The oxide or oxides of adjacent layers may be dispersed as discrete particles (34, 38) within one or more of the layers. A process for producing the article is also disclosed.

Description: This invention relates to an article comprising a a hard ceramic or cemented carbide substrate having a laminated oxide coating deposited on such a substrate. Hard ceramic and cemented carbide materials are known and are used extensively in such applications as metal cutting, tool, and bear parts, and the like. It is also known that the service properties such as wear resistance, high temperature resistance and chemical resistance of such materials may be enhanced by the application of one or more coatings of, for example, metal carbides, metal nitrides, or ceramics. Great strides have been made in improved performance of these coated substrates, for example in machining applications, by refinement of the substrate compositions and by applying various combinations of superposed layers of coating materials. However, increasingly stringent use conditions, for example use at high cutting speeds or in extremely high temperatures and/or corrosive environments, are placing increasing demands upon the performance of such materials. JP-A-59222570 discloses a process for making a composite ceramic coating film, wherein the coating consists of different layers consisting of Al₂O₃ and zirconium oxide, Al₂O₃ and hafnium oxide or Al₂O₃ and zirconium oxide - hafnium oxide. These coatings are obtained in form of a solid solution or of an appropriate mixture of the materials Al₂O₃, Zr₂O₃ and HfO₂ from a gaseous mixture composed of H₂, AlCl₃, CO and/or CO₂ and ZrCl₄ and or HfCl₄. The invention described herein and recited in the appended claims provides an article and a process in which a wear resistant laminated oxide coating of ultrathin layers of controlled composition and distribution is deposited on a hard ceramic or cemented carbide substrate, the article showing improved abrasion resistance under extreme conditions of use. A wear resistant article according to the invention comprises a hard ceramic or cemented carbide substrate body having a fully dense, adherent, wear resistant, laminated oxide coating about 0.3-20 microns thick deposited on the substrate. The coating has at least three superimposed adherent oxide layers, each about 0.1-3 microns thick, of materials independently selected from the oxides of aluminum, zirconium, and yttrium. Each layer is predominantly of a different material than the layers adjacent thereto, wherein at least one layer includes discrete particles of at least one material predominent in a layer adjacent thereto. In the preferred article according to the invention, at least one intermediate layer is deposited between the substrate body and the laminated oxide layer. The intermediate layer is about 0.5-10 microns thick and is of one or more materials selected from the carbides, nitrides, and carbonitrides of Ti, Zr, Hf, Va, Nb, Ta, Cr, Mo, W, Si, and B. In the most preferred article according to the invention, at least one of the oxide layers is about 0.1-0.5 microns thick. For a better understanding of the present invention, together with the objects, advantages, and capabilities thereof, reference is made to the following disclosure and appended claims, taken together with the drawing, of which The FIGURE is a schematic cross-sectional representation of one embodiment of an article according to the invention. The article according to the present invention may be prepared by deposition of an adherent laminated oxide-based ceramic coating on a hard ceramic substrate, for example, a monolithic or composite alumina-, silicon nitride-, or silicon aluminum oxynitride-based material or related material, or a metal cemented carbide or related material. The deposition of an oxide-based coating having three or more ultrathin layers, which is adherent to the substrate, wear resistant, high temperature resistant and resistant to chemical attack or breakdown at high temperatures depends on careful control of the process parameters. The outstanding properties of the coating are a result of the one or more additional or second phase layers of Al₂O₃, ZrO₂, or Y₂O₃, or a combination of these, alternating with Al₂O₃, ZrO₂, or Y₂O₃ first phase layers. For example, preferred coatings include alternating (ZrO₂ and/or Y₂O₃)/Al₂O₃ layers, alternating Y₂O₃/ZrO₂ layers, or alternating (Y₂O₃ plus ZrO₂)/Al₂O₃ layers, i.e. layers of an Y₂O₃-ZrO₂ solid solution, with or without Y₂O₃ and/or ZrO₂ not in solution, alternating with Al₂O₃ layers. The additional phase layers may be evenly spaced throughout the coating, or their distribution may be controlled to achieve disposition at controlled intervals throughout the coating. The alternating layers are each about 0.1-3 microns thick. Preferably, at least one layer is about 0.5 microns thick. The total thickness of the coating is about 0.3-20 microns. The preferred process for preparing the articles according to the invention, involves the use of a mixture of gases including a mixture of metal halides and other reactant gases under carefully controlled conditions to deposit by chemical vapor deposition (CVD) compounds of the metals on a substrate. A mixture of a metal halide gas, i.e. a halide of Al, Y or Zr, and other reactant gas components is passed continuously over a shaped cemented metal carbide or ceramic based substrate to deposit an oxide coating on the substrate. The other reactant gases are selected from such oxidizing gases as CO₂, H₂O and the like. The deposition temperature for the carbide substrates is about 900°-1250°C; that for the ceramic substrates is about 900°-1500°C. The pressure is between about 1.33 mbar (1 torr) and about ambient pressure. One or more carrier gases may be included in the mixture. The reactant gases, the gas flow rates and partial pressures are selected to begin to deposit a first phase oxide coating on the substrate, the first phase oxide being an oxide of Al, Y or Zr. During the deposition of the first phase oxide, at least one additional metal halide gas is mixed with the The additional metal halide is selected from the halides of Al, Y or Zr, and is different from the metal halide of the first phase mixture. The partial pressure of the additional metal halide, and the mixing length and time intervals are selected to result in the formation within the oxide coating of distinct layers of an additional metal oxide phase or phases to form the laminated structure described above with reference to the article according to the invention. This laminated structure, in which ultrathin layers of one or more additional phase oxides alternate with layers of a first phase oxide to form an adherent, fully dense, wear resistant oxide coating about 0.3-20 microns thick on the substrate, is not achievable by prior art methods, and results in articles exhibiting superior wear resistance, high temperature resistance and chemical resistance. Full control over the deposition process is achieved by control of the pulsing, i.e. the partial pressure, length and time interval of each pulse, of the metal halide gas forming the additional phase layers while maintaining continuous flow of the metal halide gas forming the first phase layers. This mixing method provides control of the thickness and distribution of the additional phase layers within the coating. The pulses of the additional metal halide or halides are controlled so that the layers of the laminated coating are each about 0.1-3 microns thick, and preferably so that at least one layer is about 0.1-0.5 microns thick. Some examples of laminated coatings according to the invention are: alternating Al₂O₃/ZrO₂ layers, alternating ZrO₂/Y₂O₃ layers, alternating Y₂O₃/ZrO₂ layers, alternating Al₂O₃/(Y₂O₃ plus ZrO₂) layers, alternating Al₂O₃/Y₂O₃ layers, alternating Al₂O₃/ZrO₂/Al₂O₃/Y₂O₃ layers. The term additional phase as used herein refers to laminates comprising layers of a first phase oxide compound alternating with one or more additional oxide layers, which may each be a single compound or more than one compound. Each layer may be oxides of a single metal or a mixture or solid solution of oxides of more than one metal, and the individual layers may be of the same or different compounds. The layers disclosed herein each normally exhibit a moderate to high degree of microscopic surface roughness, and a small number of microscopic gaps in the coverage of each layer are common. Such gaps do not appear to materially affect the properties of the coating, since the layers above and below the gap adherently bond to one another therethrough. Further, smoother and/or imperforate layers are not excluded from the invention. The introduction of an additional metal halide into the continuously flowing mixture of reactive gases results in a gradual increase and decrease in the partial pressure of the additional metal halide. This may result in the formation of discrete particles of the first phase material within the matrix of an additional phase layer and/or the formation of discrete particles of an additional phase material within the matrix of a first phase layer. Thus, each layer is predominantly of one of the above-described oxide materials, the remainder being one or more of the oxides predominant in adjacent layers. The term "predominantly", as used herein, is defined as greater than 50 volume %, i.e. each layer is composed of 50 volume % of one oxide. Preferably, each layer is 90% of one oxide. Rather than being detrimental, this can result in further improvement in the properties of the coating. The advantages of such composite layers are described in commonly assigned copending European Patent Applications Nos 88100690.2 (EP-A-0 275 977) and 88100691.0 (EP-A-0 275 978). The coatings according to the invention are fully dense, adherent, and make it possible to combine the wear-resistant properties of two or more components without the problems associated with differences in expansion coefficients and adhesion presented by coatings of thicker layers of the materials. In an alternate embodiment of the article according to the invention, a thin intermediate layer of TiC, TiN, or other carbide, nitride or carbonitride of Ti, Zr, Hf, Va, Nb, Ta, Cr, Mo, W, Si or B is deposited between the composite coating and the substrate. Such deposition may be achieved in known manner as a preliminary part of the same coating process, or in a separate, prior coating process. Similarly, for special applications, for example friction, cosmetic, wear, or thermal purposes, a thin outer layer such as TiN may be applied in known manner over the composite coating. The Figure, not drawn to scale, schematically illustrates typical coated article 10 according to the invention. As shown in the Figure, substrate 12 is a shaped WC-based composite material, and may be a cutting tool or other article requiring wear resistance under the extreme conditions described above. A thin layer 14 of TiC covers the substrate, at least in the area subjected to wear. Laminated coating 16 is deposited over TiC layer 14, and is made up of ultrathin first phase layers 18, 20, and 22 predominantly of Al₂O₃, separated by ultrathin additional phase layers 24 and 26, predominantly of Y₂O₃ and ZrO₂ respectively. An outer layer 28 of TiN is deposited over the composite layer, giving article 10 a distinctive identifying color. As described above, the gradual increase and decrease in the additional metal halide present in the reactant mixture with each pulse may result in the formation of each layer as a composite, especially near the interface of adjacent layers. This phenomenon is illustrated in the Figure, which shows discrete particles formed in each layer, of a material predominant in an adjacent layer. This will be described with respect to interface 30 between Al₂O₃ layer 18 and Y₂O₃ layer 24, but any or all of the layers of the oxide coating according to the invention may show such composite morphology. Interface 30 is represented schematically in the Figure by a smooth line, although normally some degree of microscopic roughness would be present at the interface, as discussed above. As shown in the Figure, layer 18 is made up of Al₂O₃ matrix 32 with discrete particles, as 34, of Y₂O₃ dispersed therein at and near interface 30. Similarly, layer 24 is made up of Y₂O₃ matrix 36 with discrete particles, as 38, of Al₂O₃ dispersed therein at and near interface 30. Thus both layer 18 and layer 24 are ultrathin layers of a two phase composite material, layer 18 being predominantly Al₂O₃ and layer 24 being predominantly Y₂O₃. Remaining layers 20, 22, and 26 show similar composite structure, each being predominantly of a single matrix material, with discrete particles of the matrix material of an adjacent layer dispersed therein. Layer 20 is a three phase layer having an Al₂O₃ matrix and particles of both Y₂O₃, the matrix material of layer 24, and ZrO₂, the matrix material of layer 26, dispersed therein. Layer 20 is predominantly Al₂O₃, that is greater than 50% and preferably greater than 90% Al₂O₃. Although the Figure shows layers in which the particles have formed at or near the interface, it is possible by controlling the partial pressures of the reactant gases present during the deposition of each layer, to produce one or more layers in which the particles are distributed throughout the matrix of the layer, rather than only at or near the interface with an adjacent layer. Articles including such layers, and the process by which they are produced are within the scope of this invention. EXAMPLE After rinsing of all gas lines with their respective gases for 0.5-1 hr, samples of cutting tool inserts of a silicon nitride-based composite material are coated with a layer of TiC about 3 microns thick by known techniques in a CVD reactor. An excess of preweighed zirconium metal chips is placed in a separate vessel disposed in the reactor. An excess of aluminum chips is placed in a vessel outside the reactor. The reactor is evacuated to about 13.3mbar(10 torr), then heated under low pressure, while being flushed with flowing hydrogen, to increase the outgassing before deposition. Following the deposition procedure, the reactor is cooled, at the deposition pressure and while being flushed with hydrogen, to about 300°C, then under ambient pressure and flowing nitrogen to room temperature. The deposition reaction conditions are given in Table I, below. The halide gas is Cl₂, the carrier gas for the Al and Zr halide reactions is Ar, and the other reactant gas is CO₂ with H₂ as a carrier. The Cl₂ flow rates are adjusted to give metal chloride flow rates shown in Table I. The deposition pressure is 66.5mbar (50 torr); the temperature, 1450°C. As shown in Table I, a period of Al₂O₃ deposition of 1.0 hr is carried out before deposition of the first ZrO₂ layer is begun. During the deposition of the Al₂O₃ layer, Ar gas is allowed to flow over the Zr, but the Cl₂ gas flow is shut off. The coating is deposited on the TiC underlayer as a laminated structure of alternating alumina- and zirconia-based oxide composite layers, similar to that illustrated in the Figure, but with only Al₂O₃ and ZrO₂ layers, and without the TiN layer over the oxide coating. The oxide coating is made up of four Al₂O₃ layers separated by three ZrO₂ layers. The Al₂O₃ layers each have less than 10 wt % ZrO₂ as discrete particles dispersed therein at or near the oxide layer interfaces. The ZrO₂ layers each have less than 10 wt % Al₂O₃ as discrete particles dispersed therein at or near the oxide layer interfaces. Total Flow Rate 1420 ccpm Reactant Flow Rate 1020 ccpm Volume %: H₂ 88% CO₂ 7% AlCl₃ 2.5% ZrCl₄ 2.5% Total Deposition Time 3.5 hr AlCl₃ Flow Time 0- 3.5 hr ZrCl₄ Pulse Times 1.0-1.25 hr 1.75-2.0 hr 2.5-2.75 hr

Claims: A wear resistant article comprising a cemented carbide or hard ceramic substrate body; and a fully dense, adherent, wear resistant, laminated oxide coating about 0.3-20 microns thick deposited on the substrate body, and consisting essentially of at least three superimposed, adherent oxide layers, each about 0.1-3 microns thick, of materials independently selected from the oxides of aluminum, zirconium, and yttrium, each layer being predominantly of a different material than the layers adjacent thereto, wherein at least one of the oxide layers is about 0.1-0.5 microns thick and at least one layer includes discrete particles of at least one material predominant in a layer adjacent thereto. An article according to claim 1 further comprising, between the substrate body and the laminated oxide coating, at least one adherent intermediate layer about 0.5-10 microns thick of one or more materials selected from the group consisting of the carbides, nitrides, and carbonitrides of Ti, Zr, Hf, Va, Nb, Ta, Cr, Mo, W, Si, and B. An article according to claim 1 wherein each of the layers is imperforate. An article according to claim 1 wherein at least one of the layers includes one or more gaps, the layers adjacent thereto adherently bonding to one another through the gap. An article according to claim 1 wherein the laminated oxide coating consists essentially of five layers, three imperforate first-phase layers of one of the oxide materials being interleaved with two additional phase layers of materials independently selected from the remaining of the oxide materials.

Inventors: GINDRAUX GILBERT, HINTERMANN HANS ERICH, SARIN VINOD K, GINDRAUX, GILBERT, HINTERMANN, HANS ERICH, SARIN, VINOD K.
Applicants: VALENITE INC, VALENITE INC.
Application year: 1988
Main classification: C23C16
IPC codes: C23C16, B23P15, C04B41