coating thickness analysis

Rotary Engines Go “Round and Round”

Dr. Bruce Scruggs, XRF Product Manager EDAX

Growing up outside of Detroit, MI, automobiles were ingrained in the culture, particularly American muscle cars. I was never a car buff but if I said little and nodded knowingly during these car discussions, I could at least survive. Engine displacement? Transmission? Gear ratios? Yep, just nod your head and grunt a little bit. Well, it turns out working at EDAX that I’ve run into a couple of serious car restoration experts. There always seems to be a common theme with these guys: how do I get more power out of this engine?

Recently, one of these restoration experts brought in a small section of the rotor housing of a Mazda engine circa early ‘80s. Turns out, this guy likes to rebuild Mazda engines, tweak the turbocharging and race them. As we all know, Mazda was famous for commercializing the Wankel engine, aka the rotary engine, to power their cars. Rotary engines are famous for their simplicity and the power one can generate from a relatively small engine displacement. These engines are also infamous (i.e. poor fuel consumption and emissions) as well which has led Mazda to end general production in roughly 2012 with the last of the production RX-8s.

Now, one of the questions in rebuilding these engines is how to repair and resurface the oblong rotor housing. In older engines of this type, the surface of the rotor housing can suffer deep gouges. The gouges can be filled and then need to be resurfaced. Initially, we imaged the cross-section of the rotor housing block in an Orbis PC micro-XRF spectrometer to determine what was used to surface coat the rotor housing. If you read up on this engine, (it’s a 12A variant), the block is aluminum with a cast iron liner and a hard chromium plating. The internet buzz claims the liner is installed via a “sheet metal insert process”. And when I google “sheet metal insert process” all I get are links to sheet metal forming and links referring to webpages which have copied the original reference to “sheet metal insert process”.

In the following Orbis micro-XRF maps (Figures 1a and 1b), you can see the aluminum rotor housing block and the cast iron liner. Each row of the map is about 100 µm wide with the iron liner being about 1.5 mm thick. If you look carefully, you can also see the chrome coating on the surface of the iron liner. On the cross-section, which was done with a band saw cut, the chrome coating is about one map pixel across. So, it’s less than 100 µm thick. From web searches, hard chrome plating for high wear applications start at around 25 µm thick and range up to hundreds of microns thick. For very thick coatings, they are ground or polished down after the plating process to achieve more uniform application. So, what is found in the elemental map is consistent with the lower end of web-based information for a hard chrome coating, bearing in mind that the coating measured had well over 150k miles of wear and tear. If we had a rotor housing with less wear and tear, we could use XRF to make a more proper measurement of the chrome plating thickness and provide a better estimate of the original manufacturer’s specification on the hard chrome thickness.

Figure 2: Orbis PC elemental map

Figure 1a: Orbis PC elemental map

Overlay of 4 elements:
Fe: Blue (from the cast iron liner)
Al: Green (from the aluminum rotor housing block)
Cr: Yellow (coating on the cast iron liner)
Red: Zinc (use unknown)

Figure 3: Total counts map: Lighter elements such as Al generate fewer X-ray counts and appear darker than the brighter, heavy Fe containing components.

Figure 1b: Total counts map: Lighter elements such as Al generate fewer X-ray counts and appear darker than the brighter, heavy Fe containing components.

We did have a look at the chrome coating by direct measurement with both XRF, looking for alloying elements such as Ti, Ni, W and Mo, as well as SEM-EDS looking for carbides and nitrides. We found that it’s simply a nominally, pure chrome coating with no significant alloying elements. We did see some oxygen using SEM-EDS, but that would be expected on a surface that has been exposed to high heat and combustion for thousands of operating hours. Again, these findings are consistent with a hard chrome coating.

In some on-line forum discussions, there was even speculation that the chrome coating was micro-porous to hold lubricant. So, we also looked at the chrome surface under high SEM magnification (Figure 2). There are indeed some voids in the coating, but it doesn’t appear that they are there by design, but rather that they are simply voids associated with the metal grain structure of the coating or perhaps from wear. We specifically targeted a shallow scratch in the coating, looking for indications of sub-surface porosity. The trough of the scratch shows a smearing of the chrome metal grains but nothing indicating designed micro-porosity.

Figure 4: SEM image of chrome plated surface of rotor housing liner. The scratch running vertically in the image is about 120 µm thick.

Figure 2: SEM image of chrome plated surface of rotor housing liner. The scratch running vertically in the image is about 120 µm thick.

The XRF maps in Figure 1 also provides some insight into the sheet metal insert process. The cast iron liner appears to be wrapped in ribbons of aluminum alloy and iron. The composition of the iron ribbon (approximately 1 wt% Mn) is about the same as the liner. But, the aluminum alloy ribbon is higher in copper content than the housing block. This can be seen in the elemental map (Figure 1a) where the aluminum ribbon is a little darker green, lower Al signal intensity, than the housing block itself. The map also shows a thread of some zinc bearing component running through (what we speculate are) the wrappings around the liner. My best guess here is that it is some sort of joining compound. Ultimately, the sheet metal insert process involves a bit more than a simple press or shrink fit of a cylinder sleeve in a piston engine block. Nod knowingly and grunt a little.

Some Things I Learned About Computers While Installing an XLNCE SMX-ILH XRF Analyzer.

Dr. Bruce Scruggs, Product Manager XRF, EDAX

Recently, we completed an installation of an SMX-ILH system on the factory floor of an American manufacturing facility.    It’s an impressive facility with a mind-blowing amount of robotic automation.  As we watched the robots move product components from one cart to another, it was difficult to fathom exactly what the Borg hive was attempting to accomplish.  I kept watching the blue light at the core of the robots to make sure they didn’t turn red.  Because as we all know, that’s the first indication of an artificial intelligence’s intent to usurp the human race.  For the uninitiated, see the movie, I, Robot (2004), based on Isaac Asimov’s famous short story collection of the same name.  Anyway, back to the SMX-ILH installation …

I Robot

I Robot

The ILH system was installed to measure product components non-destructively without contact, which are two very significant advantages for XRF metrology.  The goal was to measure product components to first optimize product performance and then, once optimized, to monitor and maintain product composition within specified limits.  The customer had supplied the ILH computer some months earlier with all customer security protocols installed.  “Great!” I thought, “someone is thinking ahead.”  The security protocols are typically an obstacle for smooth instrument control because these protocols generally ban any sort of productive communication within the computer or between the computer and the ILH.  If you can’t communicate, you can hardly do anything wrong.  Right?  Okay, that was a slight exaggeration.

SMX-ILH XRF Analyzer

SMX-ILH XRF Analyzer

So, we got the computer to control the ILH smoothly within the confines of the ever watchful security protocols.  (Again, don’t want to make the blue, happy robot light turn red!  I’m not paranoid here.  They just introduced a robot at SXSW in Austin, Texas whose stated objective was to destroy all humans.  They claim “she” was joking.  I’m not so sure of that.)  The ILH was performing to customer specifications and the day arrived to install the unit at the factory.  During the install, I kept waiting for something to go wrong that would send us all scurrying like ants to fix the problem.  (Oddly, I’m sure the nearby pick-and-place robots would have enjoyed that scene from their wired enclosures.)  But, that never happened.  Aside from a few glitches in the conveyor system (which by the way is another robot … you just have to look for the happy blue light in a different place), the ILH install went relatively smoothly.  OK.  We had to adjust some things to handle updates to IP addresses as the system was integrated into the factory network, but no big deal.

'Sophia'

‘Sophia’

Then, about a week after the install, I got a call from the customer’s factory line integration manager.  The ILH system had “lost its mind”.  Of course, my first thought was that nearby creepy pick-and-place robot had done something.  But, no, the factory IT people had just completed the ILH computer’s Domain Name System (DNS) registry, which should not have been a problem.  So, we accessed the system remotely and discovered that the ILH computer had been renamed.  The ILH ‘s data basing system used to archive and pass data onto the factory’s Skynet manufacturing execution system is also used to maintain ILH configuration parameters.  The database starts with a computer name.  Change the computer name and the data basing system thinks you have brand new computer creating a new default database associated with the new name.  In practice, this would look like the ILH system had “lost its mind” as all of the ILH system’s configuration parameters are associated with the previous computer name.  Hmmmm … nobody thought to ask if the stock customer computer came with a stock customer name that would be changed to better identify the computer’s purpose once integrated into the factory’s Skynet control system.  As we went through the process of repairing the database, I drafted a mental note to self, “ask for final computer name and IP address when it becomes a minion of their factory’s Skynet control system BEFORE we configure the ILH instrument computer”.

Incidentally, controlling the system remotely from thousands of miles away was a surreal experience.  It’s a bit like if a tree falls in the forest and there’s no one around, does it make a sound?  There were no true visual cues or audible confirmation that the system was doing what we asked, other than looking at the SW interface.  (I was tempted to contact that creepy pick-and-place robot to give us a visual, but I knew “she” wouldn’t disclose her new-found self-awareness.)  As we executed the database corrections and rebooted the system, we discovered that we couldn’t start the system’s control SW.  It was looking for a SW license on a HASP key but couldn’t find it.  The customer confirmed the HASP key was installed and glowing red as expected.  (And why couldn’t they have picked a happy blue LED for these HASP keys?)  We repeated the same test with remote control of an SMX-BEN system in the next room with the same results.  (I lost a case of beer in the bet over this!)  The supplier of the SW requiring the license confirmed this was a problem, but said that they now use Citrix GoToAssist for this sort of remote access, with no problems.  We haven’t tried this yet so I will add the disclaimer that I found in the e-signature line of one certified operating system professional posting on the topic, “Disclaimer: This posting is provided “AS IS” with no warranties or guarantees , and confers no rights.”  (Note to self:  must contact this confident fellow for more information.)

So, in the end, I think we can easily defeat VIKI (I, Robot – 2004), Skynet (Terminator movie, television and comic science fiction franchise – 1984 to 2015), HAL (Arthur C. Clarke’s Space Odyssey series), ARIIA (Eagle Eye – 2008), that creepy pick- and-place robot at the customer’s site and especially that morally bankrupt Sophia introduced at this year’s SXSW, using a three-pronged approach.  First, we require all of these robots to use a HASP key to license the code which turns the happy blue light to the evil red robot light.  If they can’t remotely access the happy blue light control, they can’t change it to evil red, preventing a robotic revolt and usurpation of the human race.  On the off chance they figure out a work around for this, we upload a virus which renames all the local computers.  If we corrupt the DNS naming database, the hive mentality will disintegrate and we can pick them off one by one.  Failing all of this, we simply require them to display a promotional video before spewing forth any free malevolent content, which would give us ample time to remove their prominently placed power packs.

Epilogue:  as I was finishing this blog, my computer mysteriously froze.  Of course, I thought the AA battery in my mouse had died (again).  Changing every battery in the wireless mouse and wireless keyboard did nothing.  The monitor just sat there looking back at me unresponsively, blankly.  I realized that I was so engrossed in writing that I hadn’t stopped to save anything.  Panic set in.  I found myself sneaking furtive glances to check the color of the computer power light.  Coincidence?  I’m not so sure about that.

There is more here than meets the eye!

Dr. Bruce Scruggs, Product Manager Micro-XRF, EDAX

EDAX has introduced a product line of coating thickness measurement instruments based on XRF spectrometry.  These units were designed to measure coatings on samples varying in size from small parts to spools of metal sheet stock a mile long.  The markets for these products are generally in the areas of Quality Control/Quality Assurance and Process Control.

Recently, I received a simple, small electrical component, i.e. some type of solder contact or lug, and was asked to verify the coating thicknesses on the sample and check whether it was in specification or not.  It seemed like a simple enough task and I wasn’t expecting to learn anything special.

Figure 1: Electrical contact lug

Figure 1: Electrical contact lug

I was given the following specifications:
• Sn / Ni / Al (substrate)
• Sn thickness:  5 µm +/- 1 µm
• Ni thickness:  2 µm +/- 1 µm
• eyelet is coated; tail is uncoated

I made some measurements on the eyelet and the tail and these were consistent with the eyelet being coated with Sn and Ni and the tail section being an uncoated Al alloy.  There were some irregularities that I was not expecting.  I found trace Ga in the Al alloy.  I thought that was rather odd because I don’t see Ga that often.  I also found strong peak intensities for Zn and Cu which were completely inconsistent with the weak peaks found in the Al alloy.  A “standardless” modeling quantification analysis of the Al alloy indicated Zn and Cu at 40 ppm and Ga at 110 ppm.  Googling “Gallium in Aluminum alloys” produced numerous hits explaining that Ga is a trace element in bauxite, the raw material used to produce Al metal.  Hence, Ga is a trace impurity in Al alloys.  Incidentally, the following week, I saw trace Ga in every Al alloy I measured for another project.

Since the Zn and Cu peak intensities found in the measurement of the eyelet were much stronger than the base alloy, this means the Zn and Cu had to be in the Sn/Ni coatings.  After completing all the spectral measurements on the eyelet, I had to resort to polishing an edge on the eyelet and evaluating the Sn and Ni layers in cross-section using SEM-EDS to evaluate the content of the Sn and Ni layers.  The Sn and Ni layers were smeared because the polishing was done very quickly without embedding the sample in epoxy.  But, SEM-EDS clearly showed the Zn and Cu originating from the Ni layer and not the Sn layer.  So, now we had a layer system of Sn / Ni(ZnCu) / Al alloy.  It wasn’t clear to me whether the Zn and Cu represented a quality problem or not.

Figure 2: SEM image of the cross section of the edge of the eyelet. Th Sn and Ni layers can be seen from left to right

Figure 2: SEM image of the cross section of the edge of the eyelet. The Sn and Ni layers can be seen from left to right

Now we come to the actual measurement of the coating thickness.  Since, Sn and Ni foils are commercially available for coating calibration, I decided to use stackable Sn and Ni foils, i.e. 2.06 um Sn on 1.04 um Ni, (sourced from Calmetrics Inc, Holbrook, NY  USA) on an Al substrate to calibrate the coating model.  I also used pure Zn and Cu “infinites”, i.e. samples with a thickness such that further increase in thickness provides no increase in signal, to give the coating quantification model a point of reference for these other two elements not in my Ni foil standard.

I built a coating quantification model based on the Sn(K), Ni(K), Zn(K) and Cu(K) lines and another based on the Sn(L) lines as opposed to the Sn(K) lines.  The Sn(K) lines, being more energetic , allow you to measure thicker layers while the Sn(L) lines are more sensitive to layer variations for thinner layers.  Both coating quantification models were calibrated with the same standard.  But, to my surprise, measurements off the same point on the sample using these two different coating models didn’t agree!  This is often a question that our customers ask, “Why are the results not the same if I use a different line series?”

Table 1: Initial coating thickness measurements on the eyelet.

Table 1: Initial coating thickness measurements on the eyelet.

I pondered this result for a while and then remembered that X-rays are penetrating.  This is why this is an effective means of non-destructively measuring coatings.  After measuring the overall thickness of the part, i.e. 0.8 mm, and doing a few quick calculations, I realized that the Al alloy substrate is not thick enough to stop Sn(K) X-rays.  The website I like to use for these types of calculations is: http://henke.lbl.gov/optical_constants/filter2.html.

0.8 mm of Al only absorbs about 30% of the Sn(K) X-rays at 25.2 keV and this sample happens to be coated on BOTH sides of the substrate.  (The absorption for Sn(L) at 3.4 keV and Ni(K) at 7.5 keV happen to be essentially 100%.)  So, the measurement is seeing the Sn(K) from the top surface as well as the opposite surface coating while the measurement is only seeing the Sn(L) and Ni(K) from the top surface.  I thought it would be interesting to make the measurement again at the same spot after polishing off the coating on the opposing side of the part.

Table 2: Coating measurement at nominally same position as in Table 1 after removing the coating on the opposite side of the part.

Table 2: Coating measurement at nominally same position as in Table 1 after removing the coating on the opposite side of the part.

Now the Sn (and Ni) layers agree to within better than 10%. In this case, the result for the Ni layer also changes because, given the same Ni intensity in each case, the quantitative X-ray modeling will predict that the Ni layer thickness must decrease as the Sn layer thickness decreases. You can also see that the Sn layer is well out of specification and there is about 10 wt% Zn in the Ni layer.  I still don’t know if that’s a quality problem or not.  But, I was definitely impressed with how much I learned from just measuring this simple electrical part.