Composites
Metallographic Preparation Solutions by Material
Use this guide for advice on where to start when performing metallographic preparation and analysis on composites. Included are tips for achieving the best results for your metallographic application.
Things to consider when working with composites
The differences in hardness and grinding/polishing characteristics from this range of materials mean that relief control is one of the major concerns during preparation. Pull-out and fracture problems are also very common. Sectioning frequently produces considerable damage that must be removed in subsequent preparation steps. Mounting with castable epoxy resin along with vacuum impregnation is frequently performed.
Composite Cutting Tips
All sectioning should be performed wet, with an ample flow of coolant directed into the cut. Wet cutting will produce a smoother surface finish than a dry cut. Using coolant will also guard against surface damage caused by overheating and mechanical strains. Reducing damage while sectioning is important. Mechanical damage, such as fractures can penetrate deep into the structure and make later preparation steps longer. There are two main types of cutting tools offered: Abrasive and Precision.
For composite materials typically the material more sensitive to cutting parameters should be catered to. If you have a ductile material and a brittle material that will fracture easily, cater to the brittle material.
Abrasive Cutting Tips
Abrasive blades have different bond strengths and are recommended based on the efficiency of their bond strength and abrasive type with different materials. Choosing an incorrect blade can lead to improper wear rate and possibly alter the existing microstructure. An abrasive blade is entirely made up of abrasive that is bound together by a resin. When used, blades gets smaller in size until it can no longer cut. A new blade can then be put on the sectioning piece of equipment to make additional cuts.
The size of the abrasive blade also affects cutting parameters. Larger, thicker blades remove more material but also generate more heat than a thinner blade. To reduce the heat produced by blades the feed rates should be lowered.
Abrasive wheels/blades should be selected according to the type of material and the size of part to be cut.
Precision Cutting Tips
Precision cutters are used in metallographic preparation of composites when cutting samples that are small or delicate. Precision blades don’t break down as they are being used. On the outer edge of the blade there is a section where abrasive has been bonded with metal alloy and if taken care of one wafering blade can outlast a box of abrasive blades. Wafering blades can be used with composites for more difficult cuts including making a precise cut close to a feature of interest, or when it is critical to reduce the width of a cut (kerf loss).
Additional benefits of using a precision cutter are a lower load is applied and consequently, less heat is generated, reducing the amount of damage.
Abrasive blades are also made for precision cutters. They can be used when potential for blades breaking is high. This could be if you have a difficult to clamp part, have a part that might shift during sectioning or you have more variability in the process (ex: multiple operators).
Composite Mounting Tips
Mounting metallographic samples can allow automation and increase ease of handling during the subsequent steps of metallographic preparation and examination. Intricately shaped samples can be mounted to create uniform shapes allowing automation of the sample preparation process.
Mounting samples also protects and preserves edges or surface defects during metallographic preparation. The method of mounting should in no way alter the microstructure of the specimen. Pressure and heat are the most common sources of injurious effects. Temperatures around 350oF and pressures around 4000 PSI are common for hot compression mounting.
Sample geometry and properties can determine the appropriate method for mounting composite samples. Use castable mounting when samples are thin or deformed easily. Samples such as these tend to be sensitive to high pressures. If samples are not sensitive to pressure hot compression mounting can be a time and money saver.
Hot compression mounting can damage composite samples. Castable resins with low peak temperatures are recommended.
In instances where a composite can be mounted using hot compression mounting, there are a variety of media types available. For general use, PhenoCure® is a sound choice. EpoMet® or Diallyl Phthalate exhibit better edge retention relative to PhenoCure and would be good options if this criterion is a priority. In the event that a transparent mold is of interest, consider TransOptic. Being a thermoplastic, TransOptic, when heated and held at temperatures between 105-115oC for around 20 minutes softens allowing sample removal.
For electron microscopy ProbeMet might be the conductive media you are looking for. If copper content is of interest in the sample KonductoMet® is a conductive alternative to ProbeMet.
Careful selection can ensure media meets mounting needs regarding timing, viscosity, temperature, and shrinkage. Acrylic systems can increase throughput with quick curing times between 5 and 30 minutes. Though quicker in curing, sample adhesion and viscosity of acrylics can sometimes be lacking especially with more intricate shapes. This is where epoxy systems can help. With longer working times mixed epoxy can be used with a vacuum system to help penetration levels in intricate geometries. Our epoxy systems also offer lower peak exothermic systems. Additional information can be found on our mounting page.
Composite Grinding & Polishing Tips for All Methods
Some common issues when preparing composite samples are relief, fracturing, and pull-out. To reduce the extent of relief shorter grinding steps with SiC can help. With reinforcing components, smaller than 10 microns, fracturing and pull-out from abrasives is often seen. In these cases, alternative abrasives such as alumina may be a better option than diamond.
Using Buehler’s Burst dispensing system can conserve diamond and improve consistency.
The ideal rate for the Burst dispensing system changes with the size of the platen and the polishing cloth that is being used. As a general guideline, for a platen size of 8” a burst setting of 3 is a good starting point, for sizes of 10″ and 12″ set the burst system to 4 and adjust as needed. Some experimentation may be required to determine optimal settings to ensure sufficient abrasive and wetting of the cloth. Burst dispensers are also capable of dispensing extenders simultaneously with diamond suspensions if desired.
Loads listed in grinding and polishing methods are recommendations for one 1.25″ mounted specimen. If using central force during preparation the force listed should be multiplied by the number of samples being polished. For different sample sizes, use our load conversion calculator to determine the correct load for your application.
| 4-Step Method for Polymer-Matrix Composites | ||||
|---|---|---|---|---|
| Surface | Loads [N] | Base Speed [rpm] | Relative Rotation | Time |
| CarbiMet® 320 grit | 6 [27] | 300 rpm |
|
Until Plane |
| TexMet® P with 9um MetaDi® Supreme Diamond | 6 [27] | 150 rpm |
|
5:00 |
| VerduTex with 3um MetaDi Supreme Diamond | 6 [27] | 150 rpm |
|
5:00 |
| MicroCloth® with 0.05um MasterPrep® Alumina | 6 [27] | 150 rpm |
|
1:30 |
 = Platen  = Specimen Holder *Plus MetaDi Fluid Extender as desired
|
||||
| 4-Step Method for Metal-Matrix Composites | ||||
|---|---|---|---|---|
| Surface | Loads [N] | Base Speed [rpm] | Relative Rotation | Time |
| DGD Color Yellow (35-μm Diamond) | 5 [22] | 300 rpm |
|
Until Plane |
| UltraPad with 9um MetaDi Supreme Diamond | 5 [22] | 150 rpm |
|
4:00 |
| VerduTex with 3um MetaDi Supreme Diamond | 6 [27] | 150 rpm |
|
3:00 |
| ChemoMet® with MasterMet® Colloidal Silica | 6 [27] | 150 rpm |
|
1:30 |
|
= Platen = Specimen Holder *Plus MetaDi Fluid Extender as desired
|
||||
| 4-Step Method for Ceramic-Matrix Composites | ||||
|---|---|---|---|---|
| Surface | Loads [N] | Base Speed [rpm] | Relative Rotation | Time |
| DGD Ultra Yellow (45-um Diamond) | 6 [27] | 300 rpm |
|
Until Plane |
| UltraPad with 15um MetaDi Supreme Diamond | 56[27] | 150 rpm |
|
4:00 |
| VerduTex with 6um MetaDi Supreme Diamond | 6 [27] | 150 rpm |
|
3:00 |
| VerduTex with 1um MetaDi Supreme Diamond | 6 [27] | 150 rpm |
|
2:00 |
|
= Platen = Specimen Holder *Plus MetaDi Fluid Extender as desired
|
||||
Composite Etching Tips
Due to the varying compositions of composites, etching practices should be determined by the constituents. Please review the proper pages for etching of different components.
Composite Imaging Tips
Depending on the composition and application the goal of analysis can differ. Some common goals of analysis are void measurements, delamination examination, microstructure analysis, and chemical interactions. Dimensional measurements can be taken and saved using one of the lower levels of our OmniMet® software. For other analysis and exporting needs, more advanced versions such as OmniMet Enterprise software may be required.
Composite Hardness Testing Tips
| Reference Materials for Hardness Testing | ||
|---|---|---|
| ASTM | ISO | |
| Brinell | E10 | 6506 |
| Rockwell | E18 | 6508 |
| Vickers | E92,E384 | 6507 |
| Knoop | E384 | 4545 |
| Instrumented | E2546 | 14577 |
| Conversions | E140 | 18265 |
| ASM Handbook Volume 8: Mechanical Testing and Evaluation | Hardness Testing Section | |
When the method of hardness testing is being determined, the application is assessed to find the scale that meets all needs and standards. At Buehler, our equipment supports hardness testing using four scales: Brinell, Rockwell, Vickers and Knoop. Depending on the information required and the standards to meet, the required scale can change. The table to the side lists several good reference materials for testing methods.
While working with composites several factors could affect the ability to measure the indent.
The surface finish of the specimen; flatness and scratches could hinder software auto-measuring indents. To reduce uneven surface finish, ensure that the samples are flat before starting the polishing process.
Testing composites can be difficult because of differences in reflectance between constituents. Good contrast is important when taking indent measurements. Lighting adjustments may be required for each constituent. DiaMet Full-Automatic and up have settings to allow automated illumination, and focus for each indent made.<br.
Indent tips may be hidden or obscured within pores and cracks. Ceramic constituents are especially susceptible to this. One way to abate these effects when indenting is to use a lighter load. Lighter loads not only decrease the chances of cracking but also allow for the targeting of smaller areas of material. Configure equipment with a 100x objective when measuring indents 20 microns or smaller.
Automation and documentation levels are based on the scale used. If automation is needed the different levels of DiaMet available with a tester should be discussed. Adding automation to testing processes can help to increase throughput of analysis systems and repeatability.
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