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The TE 76 Scanning Micro Hardness Tester is based on an original concept and detailed design developed by Dr. Mark Stewart, Dr. Bryan Roebuck and Dr. Mark Gee of the Centre for Materials Measurement and Technology at the National Physical Laboratory (NPL) in the UK. It is manufactured under licence by Plint and Partners.
Hardness is defined as the resistance of a material to penetration by another, harder material. The traditional Vickers, Rockwell or Brinell hardness tests involve the removal of the indenter from the surface before making a measurement of the indentation. This means that the effect of elastic deformation under the indenter is lost; the indentation represents only the plastic deformation of the material. In some materials the elastic element can be large, or total in the case of elastomers, meaning that traditional hardness measurements cannot be made.
By running a test where the force and displacement are monitored continuously during the complete indentation cycle (force increasing to a maximum and then removed), both the elastic and plastic components can be determined. The traditional hardness values can in principle be obtained without the need to measure the indentation optically. More significantly, properties of the material such as the indentation modulus and elasto-plastic hardness and dynamic properties (such as creep and relaxation) can be determined.
These measurements are described as instrumented or depth sensing hardness tests.
Traditional hardness tests rely on an optical measurement of the indentation. This can give rise to a large scatter in results because the determination of the edges of the indentation is very operator dependent. The depth sensing methods remove this operator subjectivity. This technique has been widely adopted at the nano-scale (depths less than 0.2µm) where it would otherwise be difficult to make an optical measurement of the indentation.
Hardness measurement can be defined as macro-, micro- or nano- scale according to the forces applied and displacements obtained.
Measurement of the macro-hardness of materials is a quick and simple method of obtaining mechanical property data for the bulk material from a small sample. It is also widely used for the quality control of surface treatments processes.
However, when concerned with coatings and surface properties of importance to friction and wear processes for instance, the macro-indentation depth would be too large relative to the surface-scale features.
Where materials have a fine microstructure, are multi-phase, non-homogeneous or prone to cracking, macro-hardness measurements will be highly variable and will not identify individual surface features. It is here that micro-hardness measurements are appropriate.
Traditional macro- and micro-hardness machines usually offer single point indentations with a manual or automatic stage to move between samples and make the optical measurements. Multiple indentations are required for statistical purposes. It would, however, be extremely time consuming and laborious to make a large series of hardness indentations by this method.
Many nano-scale machines can also operate at the micro-scale and incorporate automated stages. However, the precision required for the nano-indentation process means that single measurements can take many minutes and therefore any attempt to use the system for mapping a significant area of surface would take days or weeks. Such extreme high precision is not required for micro-hardness measurements.
The TE 76 Scanning Micro Hardness Tester offers automated depth sensing hardness measurement in the micro-range (0.05 to 2 N) and the low macro-range (2 to 20 N), coupled with X-Y-Z movement stages and a high speed indentation device. Indentations at these force levels will be of the order of the scale of typical material microstructures. The full automation of the indentation process and the sample movement means that large areas can be mapped in order to determine, for example:
1] The hardness of individual phases in a multi-phase system by doing a statistically significant number of indentations
2] The hardness variation in a homogeneous material to estimate the strain field
3] The hardness variation in an heterogeneous material (for example coatings, castings, welded structures, heat treated surfaces and functionally graded materials)
Such hardness maps can be obtained using the high speed indenter up to 60 times faster than nano-scale machines working at the micro-scale. For instance a matrix of 30 x 30 indents would take 2.5 hours as opposed to 6.25 days under typical operating constraints defined in the Draft ISO/CD 14577-1 (1999). This means that the mapping of a significant surface area is not only possible, but also realistic and it may be used both in a Quality Control context as well as for research and development.
With the increasing interest in and use of instrumented hardness tests, there is a clear requirement to have a recommended practice, together with certified hardness specimens and inter-laboratory data on repeatability and reproducibility.
The problems associated with the micro-hardness measurement of ceramics (due to uncertainties in the indentation depth caused by material fracture) have been addressed in a recent VAMAS Technical Working Area 3 program "Round Robin on Recording Hardness" Ullner C. and Quinn G. D., Report 33, February 1998. They identified optical glass (BK7) and silicon nitride as appropriate reference materials. A number of different testers were involved in the round robin and machine stiffness and resolution of the displacement were identified as two key issues. These findings are taken into account in the design of the TE 76.
ISO/TC 164/SC 3 Working Group "Instrumented Indentation Testing" have carried out a consultation process with participants from the USA, Germany and UK resulting in a Committee Draft ISO/CD 14577-1 (1999) "Metallic Materials - Instrumented Indentation Test for Hardness and Other Material Parameters". This defines the machine requirements, typical test methods, reference materials and data reporting options. The Draft ISO refers to single point indentation over a range from nano- to macro-scale. It does not refer to mapping a surface.
The TE 76 Scanning Micro Hardness Tester has been designed to ensure that hardness tests can be performed within the constraints recommended in the Draft ISO. Specifically this will mean that indentation speeds in this case are very much slower than those used for mapping.
The TE 76 Scanning Micro Hardness Tester offers automated depth sensing hardness measurement in the micro-range (0.05 to 2 N) and the low macro-range (2 to 20 N). It is designed to perform both high speed indentation maps of a surface as well as the much slower speed single point measurements defined in Draft ISO/CD 14557-1 (1999).
The indentation system on the TE 76 comprises a stiff linear piezo actuator mounted vertically downwards from the rigid machine frame. The Vickers diamond indenter is mounted on the underside of a small rigid cage that contains a nano-range non-contacting capacitive displacement sensor. One plate of the sensor is attached to the machine frame.
The indenter cage is rigidly fixed to a strain gauge transducer which in turn is attached to the base of the piezo actuator. This mechanical arrangement ensures that the displacement sensor is as close to the indenter as possible and does not register the force transducer deflection. This means that there is no need to correct for load cell stiffness in the force/displacement curves obtained.
The test sample is mounted on the surface of an X-Y-Z staging system with high vertical stiffness. Each axis is connected to a high resolution dc motor/drive system which may be controlled independently.
The piezo actuator has a range of 90µm, sufficient to accommodate the small machine deflections, deflection of the force transducer and the indentation itself but not sufficient to allow for a wide range of sample thicknesses.
Since the machine operation is fully automated, the Z-stage may be used intelligently both to accommodate a range of sample thicknesses (up to 25mm, the full traverse range of the stage) and to account for any unevenness of the sample surface. Thus, during a mapping routine, the software will automatically check to see if the surface is approaching or receding from the indenter and index the Z-stage to compensate.
The user may remove and relocate the sample mounting fixture accurately. This facilitates the cross-referencing of machine measurements of hardness to traditional optical determination of the projected area of the indentation by locating the sample mount on a microscope stage.
The TE 76 is supplied with a Vickers diamond indenter but may also be used with the Berkovich and other indenter shapes.
The TE 76 Scanning Micro Hardness Tester software is designed for ease of use in the Quality Control environment, but also provides the user access to all important control functions and data analysis for research and development work. The software is Windows 98/NT based and Y2K compatible.
A single indentation is specified by entering values for the following parameters:
Peak indentation Force (N) or Maximum indentation depth (µm)
Load application rate (µm/s) or Time for making the indentation (s)
The dwell time at peak force (optional parameter)
For an indentation map or scan the following additional parameters are specified:
Interval between successive indentations (normally a minimum of 5 times indentation width)
Spacing between successive series of indentations (normally the same as above)
Total number of indentations
Additional parameters can be set to define the holding of the peak force for creep tests and holding of the peak displacement for relaxation tests.
For each indentation a full loading/unloading force/displacement curve is obtained. From these curves, a number of parameters can be obtained and the user selects which are required for a particular mapping operation. A colour map is produced of the variations in the chosen parameter over the surface scanned. By clicking on one of the map points, the source force/displacement curve can be viewed. The user may elect to store all data or just the derived maps and values.
The standard TE 76 is supplied with a 20 N range transducer that provides an operating range from the high end of the micro- to the low macro-hardness. The 20 N transducer may be used below 1 N for comparison testing, but in order to run tests with the maximum error defined in the Draft ISO/CD 14577-1 (1999) the TE 76/LF lower range transducer is required.
Calibration of the normal force is carried out using NAMAS certified dead weights and the displacement by use of a NAMAS certified stepped specimen. These are provided with TE 76/CAL.
The integrity of the machine calibration and the indenter tip are checked by performing hardness tests on a NAMAS certified reference block.
| Indenter (standard): | Vickers Diamond Pyramid |
| Indenter (optional): | Berkovich Diamond |
| Indentation Device: | Low Voltage Piezo Actuator |
| Range: | 90µm |
| Force Transducer: | Strain Gauge Load Cell |
| Range: | 20 N |
| Maximum Indentation Force: | 20 N (approx. 2 kgf) |
| Resolution: | 0.001 N (1 mN) |
| Minimum Force for 1% Maximum Error: | 1 N |
| Maximum Indentation Depth: | 20µm |
| Resolution: | 0.01µm (10 nm) |
| Minimum Depth for 1% Maximum Error: | 3µm |
| Fastest Full Stroke Actuation Time: | 1 s |
| Minimum Indentation Step Size: | 0.022µm |
| Typical Distance Between Indentations: | 100µm (minimum 5 times indentation width) |
| Typical Matrix of Indentations: | 30 x 30 indents |
| Typical Time for 30 x 30 Map: | 9,000 seconds (2.5 hours) |
| Equivalent Time for ISO conditions: | 150 hours (6.25 days) |
| X-Axis Stage: | Motorised Lead-Screw Position Stage |
| Drive Type: | DC motor with integral gearhead |
| Range: | 25 mm |
| Maximum Traverse Speed: | 200µm/s |
| Positioning Resolution: | 0.1µm |
| Maximum possible deviation: | 2µm from straight line |
| Y-Axis Stage: | Motorised Lead-Screw Position Stage |
| Drive Type: | DC motor with integral gearhead |
| Range: | 25 mm |
| Maximum Traverse Speed: | 200µm/s |
| Positioning Resolution: | 0.1µm |
| Maximum possible deviation: | 2µm from straight line |
| Z-Axis Stage: | Motorised Sliding Wedge Vertical Stage |
| Drive Type: | DC motor with integral gearhead |
| Range: | 25 mm |
| Maximum Traverse Speed: | 200µm/s |
| Positioning Resolution: | 0.1µm |
| Analogue Inputs (measurements): | 16 bit resolution |
| Analogue Outputs (control): | 12 bit resolution |
| Indentation force |
| Indenter force application rate |
| Indenter displacement |
| Indenter displacement rate |
| Peak indenter displacement dwell time |
| X step increment |
| Y step increment |
| X-stage position & speed |
| Y-stage position & speed |
| Z-stage position & speed |
| Indentation force |
| Indenter displacement |
| X-stage position |
| Y-stage position |
| Z-stage position |
| Load/Displacement Curves | |
| Universal Hardness Based on Depth (HU) | |
| Universal Hardness Based on Slope (HUs) | |
| Vickers Hardness | |
| Indentation Modulus | |
| Creep | |
| Relaxation | |
| Elastic & Plastic Indentation Work | |
| Colour Maps of Hardness/Modulus | |
| Software: | Windows 98/NT compatible control, data acquisition, data analysis and microhardness mapping software. |
| PC Hardware: | Included, software pre-loaded. |
| Force Transducer: | Strain Gauge Load Cell |
| Range: | 1 N |
| Maximum Indentation Force: | 1 N (approx. 0.1 kgf) |
| Resolution: | 0.0001 N (0.1 mN) |
| Minimum Force for 1% Maximum Error: | 0.05 N |
Plint and Partners have a policy of continual improvement and reserve the right to revise specifications without notice.
Copyright © 1999 Plint and Partners Ltd.
PLINT, TE and Circle Device are Registered Trade Marks of Plint and Partners Limited.
Copyright © 2002 Plint Tribology Ltd.