cBN Polycrystallite Microstructure for High Modulus Cutting Tools
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Solution Overview
Problem
Existing cutting and grinding tools made from cubic boron nitride (cBN) polycrystallites lack sufficient resistance to elastic deformation, which compromises machining accuracy. Current cBN polycrystallites do not achieve the desired high levels of Young's modulus alongside Knoop hardness.
Innovation Solution
A cBN polycrystallite is developed with a specific microstructure comprising fine-grained, coarse-grained, and lamellar crystals of cBN, with grain sizes ranging from 10 to 250 nm and an aspect ratio less than 3. This microstructure is achieved through a two-stage heat treatment process under high pressure, ensuring a mean grain size of 80 nm or less.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional cBN polycrystallite structures are used, then Knoop hardness is achieved, but Young's modulus and resistance to elastic deformation are insufficient
Solution Approach 1:
The patent applies local quality by creating distinct crystal grain size regions within the cBN polycrystallite. Specifically, it contains fine-grained crystals (10-100 nm) for high Young's modulus and coarse-grained crystals (100 nm-10 μm) for maintaining hardness. This spatial distribution of different grain sizes optimizes both elasticity resistance and overall mechanical strength, resolving the contradiction between achieving high Young's modulus and maintaining reliability.
Solution Approach 2:
The patent effectively creates a composite microstructure within the cBN polycrystallite by combining crystals of different grain sizes (fine-grained and coarse-grained) in a single material system. This composite approach allows the fine-grained regions to contribute to high Young's modulus while coarse-grained regions maintain hardness, thereby resolving the technical contradiction between these two properties.
2Strength
If fine-grained crystal structure is used to increase Young's modulus, then resistance to elastic deformation improves, but manufacturing complexity increases
Solution Approach 1:
The patent applies preliminary action by using hexagonal boron nitride (hBN) as a pre-formed starting material with a stable hexagonal crystal structure before transformation to cubic phase. This pre-structured material provides a controlled template that facilitates the subsequent formation of the desired fine-grained and coarse-grained cBN microstructure, reducing the complexity of directly creating such a complex structure from amorphous or fine powder materials.
Solution Approach 2:
The patent utilizes parameter changes by controlling the transformation conditions (temperature, pressure, and time parameters) to achieve the desired dual-grain-size microstructure. By carefully adjusting these transformation parameters, the process converts hBN into cBN with specific fine-grained (10-100 nm) and coarse-grained (100 nm-10 μm) regions, managing microstructure complexity through controlled parameter variation rather than complex processing steps.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The resulting cBN polycrystallite exhibits Knoop hardness equal to or greater than 60 GPa and Young's modulus equal to or greater than 1082 GPa, significantly enhancing its resistance to elastic deformation and machining accuracy.
Implementation Method 1
the polycrystallite includes a fine-grained crystal of cubic boron nitride with a maximum grain size equal to or smaller than 100 nm and a mean grain size equal to or smaller than 70 nm; a plate-like crystal of cubic boron nitride having a mean longest diameter between 50 nm and 10000 nm, inclusive; and a coarse-grained crystal of cubic boron nitride having the minimum grain size larger than 100 nm and a mean grain size equal to or smaller than 1000 nm
Implementation Method 2
a method for creating a polycrystallite made of cBN by sintering a powdered material under the condition that the temperature is not higher than 2200° C. and the pressure is not higher than 25 GPa
Implementation Method 3
a method for creating a polycrystallite made of cBN by sintering a powdered material under the condition that the temperature is not higher than 2200° C. and the pressure is not higher than 25 GPa
Data Source
AI summary
A cubic boron nitride (cBN) polycrystallite includes: a fine-grained cBN crystal of 10-60 nm in grain size; a coarse-grained cBN crystal of 100-250 nm in grain size; and a lamellar crystal consisting of stacked plate-like crystals of cBN with the maximum length perpendicular to the stacking direction not larger than 1000 nm and an aspect ratio smaller than 3, where the aspect ratio is defined as the maximum length divided by a maximum length in the stacking direction, and a mean grain size of the fine-grained, coarse-grained and lamellar crystals is not larger than 80 nm. This cBN polycrystallite can be obtained by performing, under 10 GPa or lower, heating a material made of hexagonal system boron nitride to a first temperature within 1300°-1600° C., maintaining it for a first predetermined period, heating it to a second temperature within 1700°-2100° C., maintaining it for a second predetermined period, and cooling it.