Coated silicon nitride based cutting tools

Abstract: The present invention relates to coated silicon nitride based cutting tools and a method of making the same. By a carburizing heat treatment prior to the deposition of the coating an improved bonding strength of the coating is obtained. A continuous or semicontinuous transition zone of 2-10 µm thickness is formed between the substrate and the inner layer of the coating in which transition zone the glass phase is replaced by the material of the inner layer.

Description: The present invention relates to coated silicon nitride based cutting tools and a method of making the same. By a special heat treatment prior to the deposition of the coating an improved bonding strength of the coating is obtained. Silicon nitride is a highly covalent compound with a number of interesting engineering properties. An adverse effect of the strong bonding is a low self diffusivity why the material cannot be consolidated by solid state sintering. Sintering additives such as Al 2 O 3 , Y 2 O 3 and MgO are used to form a liquid with the SiO 2 which is always present on the surface of the Si 3 N 4 grains. The resulting material has a two-phase microstructure consisting of silicon nitride grains embedded in an intergranular bonding phase, which is normally a glass. During sintering the silicon nitride grains obtain an elongated shape which has a positive effect on the fracture toughness of the material. Uncoated silicon nitride cutting tools are used in cutting or milling applications of cast iron. Attempts have been made to improve the tool life of silicon nitride inserts by application of wear resistant coatings. Coated silicon nitride cutting tool inserts are known e.g. by the Sarin et al patents US 4,440,547, US 4,431,431 etc However, the coating adhesion is not always satisfactory. It is therefore an object of the present invention to provide a method of improving the adhesion of a coating on a silicon nitride based insert. It has surprisingly been found that by subjecting a silicon nitride based cutting insert to a carburizing treatment an improved adhesion of a subsequently deposited coating can be obtained. Besides improved coating adhesion in certain cases the fracture toughness of the coating is improved. Fig 2 , micrographs A1 and A2, show s in various magnifications the microstructure of the surface of a silicon nitride based cutting insert treated according to the invention. Figs 2 , micrographs B1 and B2 show s in various magnifications the microstructure of a cross section of a coated insert according to the invention. In Fig 2 X1 - Al 2 O 3 -layer. X2 - TiN-layer. X3 - silicon nitride insert. X4 - transition zone coating - insert. According to the method of the present invention a carburizing heat treatment of Si 3 N 4 -based bodies results in an improved adhesion of a subsequently deposited coating. The carburizing heat treatment takes place in a carbon containing surrounding such as a gas mixture of CH 4 (up to 25 %) and H 2 (75% or more) at a temperature of 850-1375 °C, preferably 1250-1330 °C, for 5 minutes up to 5 hours, preferably 10-60 minutes longer times being selected for lower temperatures and vice versa. The carburizing treatment can be also performed in a plasma containing a CH 4 +H 2 -gas mixture and then the carburizing time and/or temperature can be decreased. The carburizing treatment causes a change of the structure and the composition of a surface zone 2 to 200 µm, preferably 5 to 20 µm, thick. In one embodiment the bodies are carburized with a large free volume >200 % of the volume occupied by the bodies. In this embodiment SiC whiskers are formed and completely cover the surface of the body and at the same time the surface zone is partially depleted in glass phase. Various amounts of SiC are also present in the form of particles and/or semicontinuous - continuous layers between 0.3-2 µm thick as well on the body surface between SiC whiskers so to a lesser extent within the glass phase depleted zone. Whiskers grow from the silicon nitride based bodies surface at various angles. They are between 3 and 300 µm long and between 0.3 and 1.5 µm thick. The length and thickness of the whiskers depend on the carburizing conditions. Longer carburizing time gives longer whiskers and higher carburizing temperature gives thicker whiskers. In another embodiment the bodies are placed in a graphite box during the heat treatment with a small free volume between 30 % and 200% of the volume occupied by the bodies. In this embodiment only small amounts of SiC-whiskers are formed on the surface of the bodies. Depending on the carburizing conditions surface zones are formed containing SiC particles forming semicontinuous-continuous layers 0.3-2 µm thick having adjacent surface zone depleted in glass phase. SiC containing zones are easier formed at higher carburizing temperatures (higher than 1300 °C) and/or longer carburizing times. If the carburizing temperature is rather low (1200-1300 °C) or the carburizing time is short then no or less SiC particles containing interlayer is formed but the surface zone is depleted in glass phase between 2 and 10 µm depth. After carburizing, the surface zone is much rougher than before carburizing. In addition to SiC, other carbides or metals formed during carburizing treatment from the phases present within the silicon nitride substrate can be found. The formation of the SiC whiskers is faster if the carburizing is performed after application of a thin layer containing fine carbon powder to the body surface. Preferably, the carbon powder shall contain about 20 vol% of the total carbon content in the form of volatile compounds. The structure and thickness of the glass phase depleted zone is influenced by the structure and the distribution of the glass phase within the body prior to carburizing. If the glass phase is unevenly distributed the thickness of the depleted zone will vary more than for bodies with even glass phase distribution. In areas with higher contents of glass phase prior to carburizing the thickness of the depleted zone will be larger than in areas with lower concentration of the glass phase. After the carburizing treatment the bodies are coated with wear resistant coatings, containing single or multiple layers such as TiN, Ti(C,N), Ti(C,O), Ti(C,N,O) and/or Al 2 O 3 . A continuos or semicontinuous transition zone of 2-10 µm thickness between the bodies and the inner layer of the coating is formed in which transition zone the glass phase essentially is replaced by the material of the inner layer. The coating thickness is between 2 and 250 µm, preferably between 5 and 20 µm. However, bodies with long whiskers (between 15 and 230 µm long) on the surface will after the coating process contain thick coatings (between 20 and 250 µm thick) which are reinforced by in situ formed SiC whiskers. In order to decrease the tension between the coating and the silicon nitride body caused by differences in thermal expansion between the coating and silicon nitride based bodies it is in this case suitable to add to the silicon nitride based bodies up to 35 vol % wear resistant particles nitrides, carbonitrides, such as TiN, Ti(C,N) and/or oxides, such as ZrO 2 , with large coefficient of thermal expansion. Coating of the surface zones containing long whiskers can be made by conventional CVD or MT CVD technique or by infiltration with wear resistant phases by known technique, including sol-gel technique and resintering under high pressure. If the coated body shows too large a surface roughness, it is suitable to decrease the surface roughness by a mechanical treatment using known technique. Example 1 Silicon nitride cutting tool inserts-A, of style SNGA120712, S02520 were made by sintering in a conventional way. The composition (in weight-%) was: Si 54.8, N 32.9, Al 0.6, Zr 3.8, Mg 3.7 and O 4.2. Phases present were beta Si 3 N 4 (Z value=0) and tetragonal ZrO 2 . After grinding and cleaning treatments the inserts were subjected to the following carburizing heat treatment: 10 minutes at 1300 °C in 20 mbar CH 4 +980 mbar H 2 . The heat treatment was performed with the inserts placed on a graphite support with a large volume of free surrounding space (much larger than 200 % of the volume of the inserts). A metallographic examination showed that the insert surface contained large quantity of about 300 nm thick and between 10 and 15 µm long SiC whiskers growing from the silicon nitride surface at various angles. The silicon nitride surface zone was depleted from the glass phase within a 5 to 10 µm thick zone. Within this zone various amounts of SiC were found to be present. The inserts were coated with 5 µm TiN and 12 µm α-Al 2 O 3 coating. A metallographic investigation of the insert cross section showed that the areas present within the surface zone depleted in glass phase (transition zone) were infiltrated by TiN and the volume present between SiC whiskers was infiltrated by TiN. The coated inserts passed through a conventional mechanical treatment, which decreased the surface roughness of the coating. For comparison inserts of the same type were used, insert-REF A, but without carburizing treatment. Cutting test: The inserts were tested in a longitudinal turning operation without coolant. The work piece consisted of five discs of cast iron SS0125, which were pressed together in order to provide a large amount of cast iron skin, i.e. abrasive wear, and a certain degree of intermittence during each cut. Cutting speed was 750 m/min, feed 0.4 mm/rev and depth of cut 2.0 mm. Three edges per insert type were tested and the life was determined by any of the following criteria: flaking of the coating a flank wear (VB) exceeding 0.5 mm rupture too large wear in the minor cutting edge too large wear at the depth of cut The tool life of insert A (invention) was about 60% larger than that of insert REF A. Extensive flaking was observed in insert REF A. The observed improvement in tool life for the inserts A is believed to a large extent to be caused by improved bonding of the coating to the silicon nitride substrate. Example 2 Silicon nitride cutting tool inserts-B, of style SNGA120712, S02520 were sintered in a conventional way. The composition in weight % was: Si 58.9, N 37.7, Al 0.2, Zr 0.8, Mg 0.5, O 1.8 and C 0.1. Phases present were beta Si 3 N 4 (Z value=0.06). After grinding, edge rounding and cleaning treatments the inserts were subjected to a carburizing heat treatment: 60 min 1325 in 20 mbar CH 4 +980 mbar H 2 . The heat treatment was performed with the inserts placed within a graphite box with a small volume of free surrounding space (about 100 % of the volume of the inserts). Metallographic examination showed that the insert surface was depleted from the glass phase within a 2 to 5 µm thick zone. Small amounts of rather long SiC whiskers were also present. In this case the presence of unevenly distributed long SiC-whiskers has a negative effect on the coating roughness. In order to decrease the surface roughness of the coating surface the inserts prior to coating were subjected to a mechanical cleaning operation during which SiC whiskers were removed. Then the inserts were coated with a 5 µm TiN and 12 µm α-Al 2 O 3 coating in the conventional way. A metallographic investigation of the insert cross section showed that the areas within the surface zone depleted in glass phase were infiltrated by TiN. The coated inserts passed through a conventional mechanical treatment, which decreased the surface roughness of the coating. For comparison inserts of the same type were used, insert-REF B, but without carburizing treatment. The same cutting test as in Example 1 was made. Tool life of insert B(invention) was about 40% larger than that of insert REF B. Extensive flaking was observed in the reference inserts. Thus, the observed improvement in tool life for the insert B is believed to be caused by improved bonding of the coating to the silicon nitride substrate. Example 3 Silicon nitride cutting tool inserts-C, of style SNGN120712, T02520 were sintered in a conventional way. The composition of the starting mixture was: Si 3 N 4 (97 wt-%, UBE SN-E10), Y 2 O 3 (1 wt-%), Al 2 O 3 (0.5 wt-%) and Nb 2 O 5 (1.5 wt-%). Phases present were beta Si 3 N 4 (Z value=0). After grinding, edge rounding and cleaning treatments the inserts were subjected to carburizing heat treatment: 60 minutes 1300 °C in 20 mbar CH 4 +980 mbar H 2 . The heat treatment was performed with the inserts placed in a graphite box with a small volume of free surrounding space (about 100 % of the volume of the inserts). A metallographic examination showed that the insert surface was rather uneven containing large amounts of small holes present between Si 3 N 4 and SiC particles. Small amounts of short (1-3 µm long) SiC whiskers grew from the silicon nitride surface at various angles. The microstructure images of the silicon nitride surface after carburizing treatment are shown in Fig 2, images A1 and A2. The areas present between the SiC whiskers contain various amounts of SiC particles and small holes. It is believed that the reason why such a rather rough surface was obtained is the depletion of the glass phase leading to the formation of the and also the formation of the SiC whiskers and SiC particles forming a semicontinuous interlayer. The silicon nitride surface zone was depleted of glass phase in an up to 3 µm thick zone. Within this zone a certain increase of the carbon content (SiC) was observed. The inserts were coated with 5 µm TiN and 12 µm α-Al 2 O 2 coating in the conventional way. A metallographic investigation of the insert cross section showed that a transition zone was formed consisting of an intergrown mixture of TiN and carburized parts of the surface zone of the silicon nitride insert, depleted in glass phase. The microstructure images of an insert cross section after coating are shown in Fig 2, images B1 and B2 (in image B2 the transition zone is marked X4). The size of the transition zone was between 2 and 4 µm. The coated inserts passed through a conventional mechanical treatment which decreased the surface roughness of the coatings. For comparison inserts of the same type were used, insert-REF C, but without carburizing treatment. Cutting test according to Example 1 was performed with the following result. The tool life of insert C (invention) was about 60% larger than that of insert REF C. Extensive flaking was observed in the reference inserts. The observed improvement in tool life for the insert C is believed to a large extent to be caused by improved bonding of the coating to the silicon nitride substrate. The invention has been described with reference to silicon nitride materials containing Si 3 N 4 and low amounts of glass phase. Similar improvements in coatings adhesion as observed for the silicon nitride inserts can also be expected for the group of materials called sialons, including beta phase sialons with Z-value up to 4.2 and alpha+beta sialons with various proportions between alpha and beta phases. Silicon nitride based materials can also contain up to 35 vol% of other wear resistant phases such as nitrides or carbonitrides of Ti, Ta, Nb, Hf and Zr (also mixtures of these elements) and up to 35 vol % of the oxides of Zr and/or Hf.

Claims: Silicon nitride based cutting tool comprising a silicon nitride substrate including a glass phase and a single- or multilayer coating characterised in a continuous or semicontinuous transition zone of 2-10 µm thickness between the substrate and the inner layer of the coating in which transition zone the glass phase is replaced by the material of the inner layer. Cutting tool according to claim 1 characterised in that the transition zone contains SiC particles forming semicontinuous-continuous layers 0.3-2 µm thick. Cutting tool according to the claim 1 characterised in that the coating and/or transition zone contains SiC whiskers. Cutting tool according to the claim 3 characterised in that the coating containing SiC-whiskers is 20-250 µm thick. Method of making a coated silicon nitride based cutting tool comprising a substrate and a single or multilayer coating characterised in treating the substrate in a carburizing atmosphere at 850-1375 °C and thereafter depositing a single- or multilayer coating. Method according to claim 5 characterised in that said carburising treatment is performed with the substrate in a container with a small free volume between 30 % and 200% of the volume occupied by the substrate. Method according to claim 5 characterised in that said carburizing treatment is performed with the substrate in a container with a large free volume of >200% of the volume occupied by the substrate.

Inventors: MIKUS MARIAN, MIKUS, MARIAN
Applicants: SANDVIK AB, SANDVIK AKTIEBOLAG
Application year: 2000
Main classification: C04B41
IPC codes: C04B41, B23B27, B23P15