The issues of repeatability, accuracy and correlation are inherent to microhardness testing. However, by using properly maintained and calibrated equipment, trained personnel, and appropriate testing environments, testing error and variability can be reduced.
There are a large number of Vickers and Knoop testers, which are found everywhere from quality control laboratories to research labs. These hardness testing methods are useful tools in determining shallow layer hardness such as coating hardness, case depth and surface hardness. Furthermore, selective testing of specific grains or constituents cannot be performed without these tests.
Microhardness testers are highly delicate instruments. It is essential to apply only very light forces, from 10 to 1000g, and the resulting impression must be accurately measured under high magnification. Several problems are inherent to these stringent requirements.
Three Basic Problems
The problems related to microhardness testing can be separated into three categories, namely repeatability, accuracy and correlation. There are five main causes of these, which are:
- sample preparation
It is important to define the problems before discussing the causes.
- Accuracy — The ability of the instrument to read in linear fashion on recognized hardness standards (certified test blocks), and its ability to transfer this accuracy onto test specimens.
- Repeatability — A measure of how well the instrument is able to duplicate its results on recognized hardness standards.
- Correlation — The ability of the instrument to deliver results similar to those produced by another “properly calibrated” instrument; or the ability of two operators to determine the same impression using the same machine and achieve similar results.
By understanding these problems, a better insight into their causes can be gained. While there are just five major causes, there are numerous issues encompassed by each of them, the most common of which are discussed below.
Dead weights are used by microhardness testers to create force. Unlike rockwell hardness testers, these light loading devices (10-2,000gf) stack the dead weights directly on top of the indenter Hence the magnification error and several other negatives, such as knife-edges and hanging weights, are eliminated.
These force application systems are generally robust, but issues of indenter stroke can create erroneous loads.
With most machines, load application is performed at two speeds:
fast speed - this brings the indenter close to the test piece
slow speed - to contact the work and apply the load. The “stroke” of the indenter is usually set with a measuring device.
Once this “indenter tip to test surface” distance is set, the high power objective is focused on the test surface. Once properly focused, the operator will be assured that the work piece is contacted at the proper speed, and that impacting of the load has not occurred. It takes approximately 30 seconds for an instrument to make an impression, considering ASTM E384 standard dwell time of 15 seconds. This is part of the “slow” problem mentioned previously.
Even though the hardness value accuracy is not impacted by this error, in case the operator is measuring effective case depth, the distance from the edge of the sample may be wrong and ultimately result in an erroneous measurement. Figure 1 shows a Knoop indentation on a smoothly polished surface, Figure 2 shows a Knoop indentation on an etched surface and Figure 3 shows a Vickers indentation on a rougher ground surface.
Figure 1. A Knoop indentation, as it appears on a smoothly polished surface, showing good tip definition.
Figure 2. Knoop indentation, as it appears on an etched surface, showing poor tip definition - especially comparing the right side to the left
Figure 3. A Vickers indentation, as it appears on a rougher ground surface, showing potential for tip identification problems from grind marks.
No other hardness testing discipline is as impacted by the operator as microhardness. It is very rare to get two operators to exactly agree. This is often masked by users performing daily verifications of their machines.
Operators can take their time in measuring these impressions on test blocks of a known hardness whose test surfaces are typically in the optimal condition. There are two operative phrases here: “take their time” and “known hardness.” In production environments, operators are sometimes rushed to perform tests and get parts out the door. The operator knows the proper dimension of the impression and when he goes to measure it he sees that value.Proper focus is a critical factor in achieving accurate results.
Another common source of error is recording and converting results from microns to Vickers or Knoop hardness numbers. Measurement of 32.3μm can easily become 33.2μm.
The Newage CAMS (Computer Assisted Measuring System) is a modular system that can grow from a basic “click on the tips” type of a system, all the way through to full automation. This allows the user to adopt high sophistication levels that best match their needs and in case the requirements change allow the system to grow. Automatic measuring is one of the most popular features of the CAMS system. Operator influence over the measurement of impressions is eliminated and also the time-consuming process related to measuring.
Due to the light loads used in microhardness testing, vibration can be a contributor to loading accuracy. Microhardness testers should always be placed on a dedicated, level, sturdy, table that is free standing.
High magnification optical systems are utilized in microhardness testers to assist the operator in defining the small tips of the impressions. Dirt in the optical path (the ocular, optical encoder, tube, or objective) can obscure the impression or the measuring lines, making bad enough worse. A clean environment will help diminish the chances of this occurring.
Mostly samples are sectioned and mounted in bakelite or epoxy mounts prior to testing. Once sectioned and mounted, the samples are ground, sanded, and polished to provide a test surface that is free of scratches and surface texture that could otherwise interfere with the operator’s ability to discern the tips of the impression.
In case microhardness testing is to be performed on an etched sample, etch to the minimum required to visually discern the desired attribute.
Most microhardness testers are highly consistent in their ability to apply force. Excepting load cell units, this is rarely ever an issue when it comes to calibration. The measuring systems in microhardness testers vary widely, ranging from micrometer heads on an ocular, to optical encoders attached to a digital readout.
The most common error source is that induced by the operator. When surface preparation is poor, this error gets compounded. The mundane nature of measuring multiple impressions also contributes to fatigue and subsequent errors. In sporadic testing situations, it must be ensured that the operator is trained well and has the appropriate tools to prepare the test surface, the instrument is properly calibrated and in proper working condition, and that he has time to do the job properly.
Operator error can be minimized with a conscientious operator. Some level of computer testing will help when there is a need for higher volume or more frequent testing. Most microhardness testers can be adopted regardless of age. Automation eliminates any further sources of error.
This information has been sourced, reviewed and adapted from materials provided by Lloyd Instruments Ltd.
For more information on this source, please visit Lloyd Instruments Ltd.