Quality Reassurance in Material Selection
However, while a material might look good on paper and in theory, process changes, additives and contaminants can cause even the most carefully selected material to fail in production. Certain low-level elements that had not been problematic before can be the root cause of monumental interferences or product failure. For example, some ceramic manufacturers that have begun adding trace elements to enhance the applications and functionality of their products are facing issues with contamination on a molecular level where problems did not exist before.
Another problem that has occurred as ceramics have advanced is that more precise testing methods are required to keep up with this ever-changing market. Testing techniques such as thermal analysis and compositional analysis help with prototype development and prevent problems such as product failure before those problems cost the manufacturer and their customers release delays, production delays, product failures and returned goods. However, it isn't always feasible to run these tests in-house. An independent testing lab can help uncover any process changes, additives and contamination issues that occur during the manufacturing process.
Thermal AnalysisAutoclaving, firing and other processes can cause physical changes in a material that can be uncovered with thermal analysis. Thermal analysis looks for various properties such as the coefficient of expansion, melting point, etc., that are important to a ceramic product's success or failure. Prepping for a thermal analysis can be as simple as cutting the material to a size smaller than 0.01 gram from a piece as small as a pencil, or as difficult as removing a large piece from an airfoil, depending on the needs of the application.
Three different techniques that are used in thermal analysis are differential scanning calorimetry (DSC), dilatometry and thermogravimetric analysis (TGA). DSC and dilatometry are used to determine the coefficient of thermal expansion (CTE) in ceramic materials. CTE is strongly affected by the glass transition temperature, which determines phase changes. When a ceramic material undergoes a phase change, its volume also changes. For example, when water turns from a liquid to a solid to a gas, its volume goes from its heaviest form (liquid) to its lightest form (steam). Ceramic materials do change during the manufacturing process, but when they exceed the specification parameters that were initially outlined, the performance of the material can be hampered.
TGA follows the rate of reaction by approximating the weight loss of the reaction byproduct. During drying and heat treating, ceramic materials typically lose weight from escaping water or gases, and this weight loss can cause problems in the finished product. Knowing the variances of these reactions can help engineers design materials that will hold up during the reaction periods.
Compositional AnalysisAs ceramic material specifications continue to change, the type of analysis that is performed must also continue to advance. Low-level analysis requires new preparation techniques and instruments, such as direct current (DC) arc and inductively coupled plasma mass spectrometry (ICP-MS), to assess the parts-per-million levels of specific elements that might be detrimental to various products.
Unlike thermal analysis, preparing a ceramic sample for compositional analysis is more labor-intensive. In the past, chemists were challenged with the task of obtaining the material in a form that the instrument would accept for operation. As ceramic materials have become more complicated, the demand to analyze trace elements in these materials has also increased. With metal alloys, such analysis has typically been achieved with the acid digestion technique. However, many ceramic materials are unaffected by the standard acids used to break down metal alloys and quantify the elemental composition, so alternate methods are required to accomplish this task.
High-temperature fusions using fluxes such as sodium peroxide, sodium carbonate, and lithium meta- and tetra-borate are one viable alternative to acid digestion. Fusion techniques require heating the ceramic sample and flux in special crucibles to temperatures that sometimes exceed 1000°C (1832°F), which breaks down the material. These techniques are much more labor-intensive than acid dissolution and require special hoods and vessels.
Advances have been made in recent years with the use of microwave dissolution and high-temperature pressure vessels. When combined with varying acid combinations, this method can eliminate contamination and reduce the analysis time normally experienced when using standard fusion techniques.
As new ceramic materials have entered the manufacturing arena, instrumentation such as DC arc, ICP-MS, X-ray fluorescence, atomic absorption (AA) and ICP-optical emission spectroscopy (ICP-OES) have been used to perform analyses that are critical for the evaluation and quality monitoring of ceramic products. For example, while the DC-arc spectrograph has been a workhorse for ceramic analysis for many years, the method of choice for analyzing alumina, silica and zirconia materials is X-ray fluorescence due to its bulk analysis capabilities.
Instrument manufacturers are also working to meet the increasing demand for lower levels of element analysis. Perkin Elmer's development of ICP-MS with dynamic reaction cell technology (ICP-MS-DRC), which eliminates interferences, is just one example. With this new technology, laboratories are able to reach down into the parts per billion range, depending on the material matrix.
Other important instrument innovations, such as X-ray diffraction (XRD), electron microprobes and scanning electron micrographs (SEM) with energy-dispersive X-ray (EDX), are also becoming extremely important to the composition of a ceramic. XRD analyzes ceramics for phases, such as glassy phases and crystal structures, and is useful in identifying specific types of ceramics. Electron microprobes perform quantitative chemical analyses, and SEM/EDX performs a qualitative chemical analysis.
Maintaining Material IntegrityMaintaining material integrity between research and development and the manufacturing process is crucial. Performing thermal and compositional analyses at the onset of design establishes a baseline for verifying that a material meets specifications and maintains its specified characteristics through post-production processes. However, it is not always feasible to perform these analyses in-house.
Knowing that manufacturing techniques such as autoclaving and firing can change the properties of the material that was initially selected, most quality programs across the country include periodic product testing to verify that the product still meets the guidelines set during research and design. By partnering with an independent commercial laboratory, companies can stay on the cutting edge of product development without straining internal resources or compromising product quality.
About the AuthorsDavid Kluk has 32 years experience in chemical analysis, and Adrian deKrom has 22 years in thermal and physical analysis.
For more information about materials analysis, contact NSL Analytical Services at 7650 Hub Pkwy., Cleveland, OH 44125; (216) 447-1550; fax (216) 447-0716; e-mail email@example.com; or visit www.nslanalytical.com.
SIDEBAR: What to Look for in a Materials Analysis Laboratory
- Ceramic Analysis Experience. All materials are not the same, and a laboratory's experience with your material will help ensure that your samples are prepped and analyzed properly for quality results.
- Reputation. Cheaper is not always better-you usually get what you pay for. Call around and find companies that have used the lab before, and make sure the lab has a reputation for quality. If necessary, ask the lab for a list of references and contact these references yourself.
- Accreditation. Find a lab that is accredited for the type of analysis you require. Labs that have an official accreditation have undergone intensive auditing and can prove that they are qualified to perform your analysis. Two types of accreditation are Nadcap (www.nadcap.com) and ISO/IEC 17025 (www.17025.homestead.com).
- Technical Staff. The questions that need to be asked in an analysis aren't always clear. Make sure the laboratory has a technical staff member that can discuss your needs in detail to ensure that the analysis you receive provides the answers you require, and doesn't leave you holding numbers that don't mean anything.
- Fast Turnaround. Test results often mean the difference between shipped product and wasted product. Make sure the lab can meet your important deadlines-and meet them consistently-so your product isn't left sitting on the shelf or waiting for release.