A coated cutting tool, comprising: a substrate; and a coating layer formed on a surface of the substrate, wherein the coating layer includes a lower layer and an upper layer in this order from a substrate side toward a surface side, and the upper layer is formed on a surface of the lower layer, the lower layer contains a compound having a composition represented by (Al.sub.xTi.sub.1-x)N, an average thickness of the lower layer is 1.0 μm or more and 15.0 μm or less, the upper layer contains an α-Al.sub.2O.sub.3 layer containing α-Al.sub.2O.sub.3, an average thickness of the upper layer is 0.5 μm or more and 15.0 μm or less, and in grains of the α-Al.sub.2O.sub.3 layer, a proportion of grains of which a grain size is 0.05 μm or more and less than 0.5 μm is 50% by area or more and 80% by area or less.

A surface-coated cutting tool including a tool substrate containing WC crystal grains and insulating grains, and a coating layer composed of a multiple nitride of Ti, Al, and V and disposed on the surface of the tool substrate. The multiple nitride is represented by a compositional formula: Ti.sub.aAl.sub.bV.sub.cN satisfying the following relations:
0.25≤a≤0.35,
0.64≤b≤0.74,
0<c≤0.06, and
a+b+c=1
wherein each of a, b, and c represents an atomic proportion. The coating layer is characterized by exhibiting a peak attributed to a hexagonal crystal phase and a peak attributed to a cubic crystal phase as observed through X-ray diffractometry.

A surface-coated cutting tool including a tool substrate containing WC crystal grains and insulating grains, and a coating layer composed of a multiple nitride of Ti, Al, and V and disposed on the surface of the tool substrate. The multiple nitride is represented by a compositional formula: Ti.sub.aAl.sub.bV.sub.cN satisfying the following relations:
0.25≤a≤0.35,
0.64≤b≤0.74,
0<c≤0.06, and
a+b+c=1
wherein each of a, b, and c represents an atomic proportion. The coating layer is characterized by exhibiting a peak attributed to a hexagonal crystal phase and a peak attributed to a cubic crystal phase as observed through X-ray diffractometry.

A method for forming an oxidation protection system on a composite structure may comprise: applying a ceramic layer slurry to the composite structure, wherein the ceramic layer slurry comprises aluminum and silicon in a solvent or carrier fluid; and heating the composite structure in an environment comprising nitrogen gas and oxygen gas to form a ceramic layer on the composite structure, wherein the ceramic layer comprises aluminum nitride and alumina.

A method for forming an oxidation protection system on a composite structure may comprise: applying a ceramic layer slurry to the composite structure, wherein the ceramic layer slurry comprises aluminum and silicon in a solvent or carrier fluid; and heating the composite structure in an environment comprising nitrogen gas and oxygen gas to form a ceramic layer on the composite structure, wherein the ceramic layer comprises aluminum nitride and alumina.

A surface-coated boron nitride sintered body tool is provided, in which at least a cutting edge portion includes a cubic boron nitride sintered body and a coating film formed on a surface of the cubic boron nitride sintered body. The coating film includes an A layer and a B layer. The A layer is formed of columnar crystals each having a particle size of 10 nm or more and 400 nm or less. The B layer is formed of columnar crystals each having a particle size of 5 nm or more and 70 nm or less. The B layer is formed by alternately stacking two or more compound layers having different compositions. The compound layers each have a thickness of 0.5 nm or more and 300 nm or less.

A surface-coated boron nitride sintered body tool is provided, in which at least a cutting edge portion includes a cubic boron nitride sintered body and a coating film formed on a surface of the cubic boron nitride sintered body. The coating film includes an A layer and a B layer. The A layer is formed of columnar crystals each having a particle size of 10 nm or more and 400 nm or less. The B layer is formed of columnar crystals each having a particle size of 5 nm or more and 70 nm or less. The B layer is formed by alternately stacking two or more compound layers having different compositions. The compound layers each have a thickness of 0.5 nm or more and 300 nm or less.

A cBN sintered body has 40%-85% cBN by volume and 15% to 60% binder phase by volume. and inevitable impurities. The binder phase has an Al compound including Al and at least one element selected from N, O and B, and a Zr compound including Zr and at least one element selected from C, N, O and B. The Zr compound includes ZrO, or ZrO and ZrO.sub.2. In an X-ray diffraction, where a peak intensity of a (111) plane of the ZrO is I.sub.1, a peak intensity of a (101) plane of tetragonal ZrO.sub.2 is I.sub.2t and a peak intensity of a (111) plane of cubic ZrO.sub.2 is I.sub.2c, a ratio of the intensity of I.sub.1 to total intensities of I.sub.1, I.sub.2t and I.sub.2c is 0.6-1.0, and an average grain size of the Al compound is 80 nm-300 nm.

A cBN sintered body has 40%-85% cBN by volume and 15% to 60% binder phase by volume. and inevitable impurities. The binder phase has an Al compound including Al and at least one element selected from N, O and B, and a Zr compound including Zr and at least one element selected from C, N, O and B. The Zr compound includes ZrO, or ZrO and ZrO.sub.2. In an X-ray diffraction, where a peak intensity of a (111) plane of the ZrO is I.sub.1, a peak intensity of a (101) plane of tetragonal ZrO.sub.2 is I.sub.2t and a peak intensity of a (111) plane of cubic ZrO.sub.2 is I.sub.2c, a ratio of the intensity of I.sub.1 to total intensities of I.sub.1, I.sub.2t and I.sub.2c is 0.6-1.0, and an average grain size of the Al compound is 80 nm-300 nm.

An object of the present invention is to provide a novel technique capable of suppressing the occurrence of cracks in an AlN layer.

The present invention is a method for manufacturing an AlN substrate, the method including: an embrittlement processing step S10 of reducing strength of a SiC underlying substrate 10; and a crystal growth step S20 of forming an AlN layer 20 on the SiC underlying substrate 10. In addition, the present invention is a method for suppressing the occurrence of cracks in the AlN layer 20, the method including the embrittlement processing step S10 of reducing the strength of the SiC underlying substrate 10 before forming the AlN layer 20 on the SiC underlying substrate 10.