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Impact Toughness And Thermal Shock Toughness Of Superhard Abrasives

May 12,2023

The impact index (TI) and thermal shock index (TTI) of superhard abrasive are the most popular indicators to measure the quality of superhard abrasive products in the world. Ultra hard abrasives and products are subjected to both impact and wear during actual use, and the abrasive particles of ultra hard products are subjected to high temperature during production and use. Therefore, when used for testing, it is necessary to simulate the impact, wear, and high temperature forms of the abrasive particles during actual production and use, in order to objectively reflect the quality of ultra hard abrasives.

1. Definition
Impact toughness (TI):
At room temperature, using impact toughness testing equipment, a certain amount of superhard abrasive particles are placed in a specific impact tank with a specific size steel ball. After the specified number of impacts within a specified time, the percentage of unbroken abrasive particles obtained is TI.
Measuring the impact toughness (impact strength) of superhard abrasives can more objectively reflect their actual use effect and quality than measuring their single particle static pressure strength. At the same time, measuring impact toughness also has advantages such as good sample representativeness and small measurement error.
If the high temperature conditions during the use of superhard products are taken into account, the thermal shock toughness is an index to measure the heat resistance of superhard abrasives.

Thermal shock toughness (TTI):
The TTI value is obtained by repeating the TI operation after a certain amount of superhard abrasive particles are heated at a high temperature of 1100 ℃ for a specified time in a protective atmosphere.
The numerical value represents the level of thermal stability of the abrasive. The principle of thermal shock toughness measurement is to add a heating link to the impact toughness. The superhard abrasive is heated in a special equipment filled with inert gas (such as argon), and then its impact toughness is measured.
The impact toughness and thermal shock toughness testing technology is currently widely used in the field of superhard material, and the technology is relatively mature. Before the 1990s, China’s superhard abrasive testing technology has always used the single particle compressive strength testing method, which has the advantage of simple testing method, but this method has large testing errors and poor repeatability, and its testing mechanism is also difficult to reflect the stress state of the abrasive under dynamic conditions.
Later, China developed an impact toughness tester and a special TTI heater for measuring the thermal shock toughness of superhard abrasives according to the actual testing needs, and formulated corresponding testing methods and standards to control the quality of superhard abrasive production. Due to the diverse forms and performance of impact instruments and TTI heating furnaces both domestically and internationally, this article will only provide a brief scientific introduction for everyone to understand and learn from.

2. Testing method
There are two main methods for characterizing impact toughness testing techniques based on their test results, which are different domestically and internationally:
(1)Take a certain amount of superhard abrasive basic particles and load them into the sample tube together with steel balls of a certain specification and quality. Impact them on the impact tester at a specified frequency for a certain number of times (or time), and then screen them again to obtain the unbroken rate (the ratio of the mass of the material on the screen after impact to the original mass), expressed as a percentage, as its impact toughness value.
(2)Take a certain amount of superhard abrasive basic particles together with a certain specification and quality of steel balls and load them into the sample tube. After a certain frequency and a certain number of times (or time) of impact on the impact tester, screen them to determine the unbroken state. Then, through verification and conversion, the impact toughness of the sample is characterized by the number of times (or time) of impact when the unbroken rate is 50%.

The first testing method is simple and easy to operate, and is widely used by major companies both domestically and internationally. However, it requires adjusting the impact frequency (or time) of the impact instrument as the abrasive grade changes to ensure the accuracy of the experiment (for example, the sensitivity is highest between 40% and 60% of the unbroken rate ratio).

The second testing method is relatively complex in operation, but it can compare various grades of superhard abrasives under the same conditions. The disadvantage is that in order to obtain a test result, at least two experiments need to be conducted. This method is applied in FEPA standards, former Soviet Union standards, and Chinese standards.

The test method of thermal shock toughness varies from country to country or company, mainly in terms of heating rate, holding time, protective gas and heating temperature. 900 ℃ and 1100 ℃ are the most used heating temperatures for diamond abrasives, and 1000 ℃ and 1100 ℃ are the most used heating temperatures for CBN abrasives. It is also used at 950 ℃, 1150 ℃, and 1200 ℃, and the main protective gases used are argon, hydrogen, and nitrogen.

For example, to test the thermal shock toughness of diamond particles, a certain quality of diamond particles and steel balls are taken and loaded into the sample tube, heated to 900 ℃ or 1100 ℃, and then subjected to shock at the frequency of 2000 r/min. Calculate the unbroken rate after impact. Characterize the impact toughness of the sample by the number of impacts at a 50% unbroken rate.

It should be noted that the method of measuring impact toughness here is different from traditional methods, mainly due to the small size of abrasive particles, which cannot be measured according to the traditional method of stamping large volume materials. Thermal shock toughness is a widely used performance index in the world, especially for high-grade diamonds, which is one of the essential quality indexes to determine their use performance.

3. Testing equipment
The working principle of the impact toughness tester is mainly through the reciprocating motion of the sample tube, driving the steel ball to impact the sample in the test tube, and the abrasive is broken due to the impact. The magnitude of the impact force is determined by the speed of the reciprocating motion of the sample tube and the mass of the steel ball, so the abrasive is broken under the combined effect of the frequency of impact, the amplitude of the sample tube, and the mass of the steel ball. From this, it can be seen that under the same impact conditions, the greater the unbroken rate, the better the toughness of the abrasive.
The working principle of thermal shock toughness is mainly to put the super hard abrasive into the TTI heating furnace, exhaust the air in the furnace, introduce the protective gas, heat it according to the predetermined temperature rise curve, cool it, and then measure the impact toughness.
There are two heating methods for TTI heating furnace: resistance wire heating and silicon carbon tube heating, which can be divided into bell type and tube type in terms of shape. The bell type TTI heating furnace uses molybdenum wire as the heating element, with a maximum heating temperature of 1400 ℃. It rises from room temperature to 1300 ℃ for about 2 hours. The heating rate is slow and the furnace chamber space is large. The performance of a tube type heating furnace using molybdenum wire as the heating element is the same as before, with limited furnace chamber and less material burned in each furnace. The commonly used heating furnace in China is a tubular heating furnace that uses silicon carbon tubes as the heating element, with a fast temperature rise rate and the ability to burn 1-2 samples each time.

△ A certain type of impact toughness testing device

The values of TI and TTI reflect the impact resistance and heat resistance of superhard abrasives, and the difference between TI and TTI reflects the thermal stability of the abrasive particles.

If TI represents the possible strength of superhard abrasive particles, TTI represents the actual strength of abrasive particles in use. It should also be noted that the difference between TI and TTI is an important indicator for evaluating the performance of superhard abrasive particles.

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