Coated silicon nitride based cutting tools

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 .mu.m 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.

BACKGROUND OF THE INVENTION
 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 which is why the material cannot be consolidated by solid
 state sintering. Sintering additives such as Al.sub.2 O.sub.3, Y.sub.2
 O.sub.3 and MgO are used to form a liquid with the SiO.sub.2 which is
 always present on the surface of the Si.sub.3 N.sub.4 grains. The
 resulting material has a two-phase microstructure 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 the cast irons. Attempts have been made to improve the
 tool life of the silicon nitride insert by application of wear resistant
 coatings. Coated silicon nitride cutting tool inserts are known, e.g., by
 the Sarin et al. U.S. Pat. Nos. 4,440,547, 4,431,431, etc.
 However, the coating adhesion is not always satisfactory.
 OBJECTS AND SUMMARY OF THE INVENTION
 It is an object of this invention to avoid or alleviate the problems of the
 prior art.
 It is further an object of this invention to provide a method of improving
 the adhesion of a coating on a silicon nitride based insert.
 In one aspect of the invention there is provided a silicon nitride based
 cutting tool comprising a silicon nitride containing substrate including a
 glass phase, a single or multilayer outer coating and a continuous or
 semicontinuous transition zone of 2-10 .mu.m thickness between the
 substrate and the inner layer of the outer coating in which transition
 zone the glass phase is replaced by the material of the inner layer.
 In another aspect of the invention there is provided a method of making a
 coated silicon nitride based cutting tool comprising a substrate and a
 single or multilayer outer coating comprising treating the substrate in a
 carburizing atmosphere at 850-1375.degree. C. and thereafter depositing
 the single or multilayer coating.

EXAMPLE 1
 Silicon nitride cutting tool inserts A, of style SNGA120712, S02520 were
 produced 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.sub.3 N.sub.4 (Z-value=0) and tetragonal ZrO.sub.2. After
 grinding and cleaning treatments, the inserts were subjected to the
 following carburizing heat treatment: 10 minutes at 1300.degree. C. in 20
 mbar CH.sub.4 +980 mbar H.sub.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 a
 large quantity of about 300 nm thick and between 10 and 15 .mu.m long SiC
 whiskers growing from the silicon nitride surface at various angles. The
 microstructure images of a silicon nitride surface after carburizing
 treatment are shown in FIGS. 1A and 1C. The silicon nitride surface zone
 was depleted from the glass phase within a 5 to 10 .mu.m thick zone.
 Within this zone, various amounts of SiC were found to be present, The
 inserts were coated with 5 .mu.m TiN and 12 .mu.m .alpha.-Al.sub.2 O.sub.3
 coatings. 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 microstructure images are
 shown in FIGS. 1B and 1C (in image 1D the transition zone is marked X4).
 The total coating thickness was TiN(+SiC whiskers)-20 .mu.m and Al.sub.2
 O.sub.3 -12 .mu.m. The coated inserts passed through a conventional
 mechanical treatment, which decreased the surface roughness of the
 coatings.
 For comparison, the same types of inserts 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 without carburizing heat treatment. Extensive flaking was
 observed in insert REF A without carburizing heat treatment. The observed
 improvement in tool life for the insert A is believed to a large extent 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.sub.3 N.sub.4 (Z-value=0.06). After grinding, edge rounding and
 cleaning treatments, the inserts were subjected to carburizing heat
 treatment: 60 min 1325.degree. C. in 20 mbar CH.sub.4 +980 mbar H.sub.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 .mu.m thick
 zone. The microstructure images of the silicon nitride surface after
 carburizing treatment are shown in FIGS. 3A and 3C. 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 .mu.m TiC and 12 .mu.m .alpha.-Al.sub.2
 O.sub.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 microstructure images
 of the insert cross-section are shown in FIGS. 3B and 3D (in image 3D the
 transition zone is marked X4). The coated inserts passed through a
 conventional mechanical treatment, which decreased the surface roughness
 of the coatings.
 For comparison, the same type of inserts 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 without carburizing heat treatment. 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, S02520 were
 sintered in a conventional way. The composition of the starting mixture
 was Si.sub.3 N.sub.4 (97 wt-%, UBE SN-E10), Y.sub.2 O.sub.3 (1 wt-%),
 Al.sub.2 O.sub.3 (0.5 wt-%) and Nb.sub.2 O.sub.5 (1.5 wt-%). Phases
 present were beta Si.sub.3 N.sub.4 (Z-value=0). After grinding, edge
 rounding and cleaning treatments, the inserts were subjected to
 carburizing heat treatment: 60 minutes 1300.degree. C. in 20 mbar CH.sub.4
 +980 mbar H.sub.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 is rather uneven containing large amounts
 of small holes present between Si.sub.3 N.sub.4 and SiC particles. Small
 amounts of short (1-3 .mu.m long) SiC whiskers growing from the silicon
 nitride surface at various angles were also present. The microstructure
 images of a silicon nitride surface after carburizing treatment are shown
 in FIGS. 2A and 2C. 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 rough surface was obtained is the depletion of the glass
 phase leading to the formation of the holes 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
 .mu.m thick zone. Within this zone, a certain increase of the carbon
 content (SiC) was observed. The inserts were coated with 5 .mu.m TiN and
 12 .mu.m .alpha.-Al.sub.2 O.sub.3 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 FIGS. 2B and 2D (in image 2D the
 transition zone is marked X4). The size of the transition zone was between
 2 and 4 .mu.m. The coated inserts passed through a conventional mechanical
 treatment which decreased the surface roughness of the coatings.
 For comparison, they same type of inserts 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 without carburizing heat treatment. Extensive flaking was
 observed in the reference inserts. Thus, 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.sub.3 N.sub.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. SiAlON 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 Zr and/or Hf.
 The principles, preferred embodiments and modes of operation of the present
 invention have been described in the foregoing specification. The
 invention which is intended to be protected herein, however, is not to be
 construed as limited to the particular forms disclosed, since these are to
 be regarded as illustrative rather than restrictive. Variations and
 changes may be made by those skilled in the art without departing from the
 spirit of the invention.