X-Ray Fluorescence Measures Coating Thickness

May 5, 2003
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Balzers Inc. is a coating company, plain and simple. To paraphrase a commercial, they don't make the cutting tools that bore through hard metal; they make them cut longer. They don't make the gears that fit in the palm of a hand and help propel heavy-duty transmissions; they make them run smoother. And for a cutting tool or component to do these things, it must be coated to the correct thickness.

Using a physical vapor deposition (PVD) process, Balzers applies thin-film, wear-resistant coatings to cutting tools, forming tools, molds, dies and precision components. A customer supplies the blanks, and Balzers coats them, like a chef preparing a meal in which the customer supplies half of the ingredients.

Alphabet Soup
The coatings are an alphabet of metallic compounds--TiN, CrC, WC/C--and the recipes used depend on what the user will do with the coated product. Titanium nitride (TiN) works well for machining iron-based materials, while chromium nitride (CrN) is more appropriate for machining copper alloys. Chromium carbide (CrC), with its high temperature oxidation resistance, is best for coating cores and molds used in die casting. Tungsten carbide/carbon (WC/C), with its low coefficient of friction resulting from a mix of hard tungsten carbide particles and a softer amorphous carbon matrix, is designed to coat and protect highly-loaded precision components, gears and gear drives, engine components and hydraulic pumps and compressors.

Each application has its own thickness specification--not too much, not too little.

Balzers is a Liechtenstein-based company that has coating centers in eight U.S. locations, and each location uses X-ray fluorescent technology to measure coating thickness. One such center is located next to the company's U.S. headquarters, just north of Buffalo, in Amherst, NY. The center coats cutting tools and components and also houses the Strategic Business Unit (SBU) Component Center.

The SBU Component Center has a fully equipped metallurgical lab (Met Lab) overseen by Chief Metallurgist Mary Jane Hornung. Here, Hornung, along with her assistant, materials technician Karen DiPalma, prepare coating thick-ness standard sets, establish calibration curves, train quality coordinators and technicians, and perform failure analysis. Coating thickness is one of the most important tests that Balzers performs, because of the key role it plays in part performance, says Hornung.

"If the coating thickness is below a specification level, customers may not get the performance that they need," Hornung says. "PVD coatings have a certain amount of residual stress, and as coating thickness increases, there is an increase in the residual stress. If coating is too thick, it can flake off a cutting tool, particularly one with a very sharp edge."

The X-ray fluorescence method doesn't measure thickness per se. Instead, it measures the intensity of the X-rays emitted from the coating when it is exposed to a primary X-ray beam, says Hornung. This intensity per unit time is called count rate. The higher the count rate, the thicker the coating. "For example, if you have a 1-micron TiN coating on a part and you excite it with an X-ray, that coating will give off a certain amount of signal. A 2-micron coating will theoretically give you twice as much signal."

In its U.S. coating centers, Balzers uses X-ray fluorescent instruments made by Fischer Technology Inc. (Windsor, CT). Balzers has several XRF models throughout its coating centers, and the Met Lab uses one of the newest models, the XDVM-W, for testing, failure analysis and training.

Coating plungers
Al Horack uses the Met Lab XDVM-W for measuring coating thickness. He is the Multi-Chamber System (MCS) coordinator who inspects components that have been coated in the MCS coating vessel. The coating he applies and tests is a Tungsten Carbide/Carbon recipe, which the company sells under the brand name Balinit C. The com-pany's Web site says that WC/C is a popular coating because of its low coefficient of friction that makes it "ideal" for applications such as fuel injection system components, gear and gear drives, engine components and hydraulic pumps and compressors. A broad range of steels with tempering temperatures below 500 F can be coated without concern for softening or distorting.

Horack receives uncoated components--specifically, plungers that are used in diesel fuel-injection systems--which are then demagnetized, cleaned and tested for hardness. Hardness is measured based on the Rockwell C scale (HRC), using the Wilson 2000 tester by Wilson Instruments, a division of Instron Corp. (Canton, MA). This measurement gives a base hardness number that will be verified at the end of the coating cycle to ensure that the coating process didn't soften the plunger.

The parts are fixtured and loaded into the multi-chamber coating vessel. The appropriate coating recipe program is selected and the thin-film is deposited over the blanks. After the coating process, another follow-up hardness test is conducted around the circumference of the plunger. If it is determined that the hardness level has remained unchanged, a sampling of parts is checked for thickness.

Parts are randomly selected from each coating level in the vessel to check for thickness. Thirty plungers can be measured at a time, an improvement over a previous system that measured thickness one part at a time. The parts are staged on a white Teflon block with multiple v-notches that hold the plungers in place. The proper calibration curve, which is specific to the material for the plunger and coating type, is selected for measurement.

The coated plungers are then placed on the XYZ stage inside the benchtop XRF unit. An X-ray tube sits above the staged part and it sends the X-ray into the part. The signature X-ray fluorescence for the coating is captured by a detector that converts the X-ray fluorescence into an electrical impulse and converts the impulse to a coating thickness number. This thickness data is then checked against the specification limits.

Horack turns the plunger three times in 90-degree increments until the part has been tested a full 360-degrees. The thickness must be within a small tolerance window; Balzers' coating process is able to hold tolerances within 1 micron or less, depending on requirements.

Call the customer
"We want to know if we have an even coating around the outer diameter of the part and that there are no failures," says Horack. "If one side is low, then another side would probably be higher, and if that would happen, I would put the level on hold and call the customer."

This summer, Balzers upgraded Horack's data collection ability by adding a proprietary software system called Miniwedge that is attached directly to the XRF. "I used to export data from the XRF into an Excel file and add up different readings for the different elements in the coating," he says. "Miniwedge automatically puts the readings together and provides a summary of the Cpk values, and other values, and then gives it to us in a graphic chart at the bottom of each page to see the change."

Coating thickness testing of gears is a similar process to testing plungers, says Bryan Wollenberg, who is the quality coordinator responsible for inspecting gears at the Amherst facility. Before coating, the gears are demagnetized, cleaned and hardness tested, in this case by taking a 45N Rockwell superficial hardness test. After coating, the gears are again hardness tested, and an adhesion test using the HRC scale is taken.

Thickness tests are then conducted on the crests and flanks of the gear teeth using the XRF.

To measure the coating on the crests of the teeth, the gears are slid over a fixture consisting of a long pole with caps on the ends. The calibration curve for the particular gear is selected. After each measurement, which takes about 20 seconds, the pole is manually turned until the entire gear base is checked. The gears are then placed on a different fixture so the coating thickness on the flanks can be measured.

"This flank measurement is probably more important because that is where the two gears are coming together," says Wollenberg. The thickness on the crests and flanks don't have to be identical, but it must be close. "We don't want to see anything way out of whack," he says. "But they will slightly vary because of the sputtering coating process."

If a product is out of specification, which Wollenberg is quick to point out is a rare occurrence--"only if there was a power failure or something like that would we have a problem such as this"--then the out-of-tolerance parts are scrapped and a "concern report" is filled out.

Failure faults found
The Met Lab examines and tests parts that have come in from the field with problems. When tools don't perform as expected, DiPalma and Hornung try to figure out why. Preliminary testing usually involves visual exam, adhesion test and coating thickness measurement.

"It is important to measure coating thickness early in a failure analysis," says DiPalma, who Hornung calls an XRF guru. "We want to make sure we put the right amount of the coating on."

DiPalma often doesn't know what material was used to make the tool or component. The XRF has a material library in which a spectra of different materials can be stored. A spectrum is like a fingerprint. And the XRF matches the spectrum of the unknown material with a spectrum stored in the material library.

The XRF has a number of other features. "One of the features that I use a lot is called spectrum analysis," says DiPalma. "We put a piece on the XRF and take a spectrum measurement. The spectrum has peaks and each peak represents an element. You are able to choose an element from a list and match it to a peak on the spectrum." In addition, the spectrum analysis has a feature that detects the percent of an element in that part, which is another method of identifying a material.

DiPalma uses the spectrum analysis to test brazed parts. By analyzing the brazed joint, she can determine if the correct brazing alloy was used for that part. Some brazing alloys melt or out-gas at coating temperatures. This can cause poor coating adhesion and loss of joint strength.

Another test is called elements analysis. One problem for which elements analysis is particularly effective is a phenomenon that Hornung calls cobalt leaching, which can be a problem with carbide cutting tools. One type of elements analysis, called the percentage of cobalt in tungsten carbide, determines the percent of cobalt in a carbide substrate. Leaching is determined by measuring the difference between surface and subsurface cobalt percentages. The greater the difference in cobalt percentages, the greater the possibility that carbide was leached. This is important to Balzers because it is difficult for a coating to adhere to a leached surface. This can result in coating loss.

Standards
The key to measuring any coating thickness is to have a good calibration curve established in the XRF, says Hornung. "To have a good calibration curve, you have to have good standards."

To develop these standards, the Met Lab uses flat coupons, small round discs that look like hockey pucks. The coupons or parts are referred to as substrates. They are fixtured and placed in a coating vessel so a different thickness is generated on each coupon or part. Two to three Calo wear scars are placed on each coupon or part. A Calo wear scar is produced by rotating a hardened steel ball to which a diamond slurry has been added against a coupon or part until the coating is worn through. This creates a small "crater" that on a flat coupon is round. In the center of the "crater" is the exposed substrate. Surrounding the crater is the coating. An optical microscope from Nikon Inc. (Mel-ville, NY) and special software called analySIS, from Soft Imaging System (Munster, Germany), is used to measure the coating thickness. This thickness value is assigned to the coupon or part. The coupon or part is now called a standard.

Hornung uses three to five standards for a given standard set. The set should include one standard that falls below the required measurement range, one that falls above and one or more in between the upper and lower limits of the range. "I think one of the biggest things that we recognize is that what we get out of X-ray fluorescence instrument is only as good as what gets put in." Hornung concludes.

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