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Mechanical Properties of Certain Ceramic Fasteners

Introduction

Ceramic materials excel where metallic or plastic substitutes simply cannot provide the necessary performance. High wear situations, electrical or thermal insulating applications, corrosive environments, or instances where magnetism, biocompatibility, or outgassing are concerns for many cutting-edge fields. As the extreme performance demands of oil and gas extraction, chemical detection, or deep space exploration (to name a few) go un-met, engineers and designers are turning to ceramic materials to address the performance gaps. Ceramco’s line of stock solid ceramic fasteners are the coupling solution when the standard falls short.

Ceramco’s solid ceramic fasteners continue to generate considerable interest since they were offered commercially in 1990. With increased interest and exposure has come an increasing desire for mechanical properties (such as recommended installation or service torque and tensile strength) from customers seeking to utilize these fasteners in their applications. To better understand the capabilities of our products and meet the demands and requests of our customers who are becoming more technically advanced in aggregate, Ceramco has obtained unbiased, third-party data based on industry standard test methodologies.  Initial data was used for the below listed values. As testing continues this data will help drive and guide material and process development within the company and expand our product offerings.

Additional preliminary research and prototyping of Ceramic Matrix Composites (CMCs) and Zirconia Toughened Alumina (ZTA) fasteners has already begun.

Sampling

An initial 800 piece lot of high purity alumina (99.8% Al2O3) fasteners was submitted to an independent laboratory to begin testing, with supplementary lots of varied materials (stabilized ZrO2) and sizes forwarded thereafter. Submitted lots were evenly divided for destructive Tensile and Torsion testing. Sizes and compositions tested to-date are as follows:

SizeLengthTypeMat’l. Tested
5/16"-183.000"Hex BoltAlumina
1/4"-203.125"Hex BoltAlumina/Zirconia
#10-322.063"SHCSAlumina
#8-322.000"Hex BoltAlumina
#6-321.500"Hex BoltAlumina
#4-401.063"Hex BoltAlumina
M5 X .840mmHex BoltAlumina/Zirconia
M4 X .740mmHex BoltAlumina/Zirconia
M3 X .525mmPHPAlumina
M2 X .410mmHex BoltAlumina

Test Method

Engineered ceramics of the high-purity oxide type have high relative hardness and correspondingly low ductility. As such, test results would show no definable separation between proportional and elastic limits, thereby limiting the utility of NDT. Acknowledging these inherent issues from the outset, Ceramco elected that all testing would be destructive.

Testing was conducted per the methodologies outlined by ASTM F606/F606M Standard Test Methods for Determining the Mechanical Properties of Externally and Internally Threaded Fasteners, Washers, Direct Tension Indicators and Rivets.

Tested sizes were selected based on historic customer demand (though length was determined by the test facility based on fixturing needs) and tailored to ensure that the range of tested sizes would allow for reasonable extrapolation for uncommon and non-standard sizes. Test specimens were randomly selected from current and former production lots, packaged to maintain randomization, labeled for size and material and delivered to the test facility.

Test Summary

In general, obtained data well represented the normalcy and distribution anticipated of engineered ceramic materials tested in tensile and torsional modes. Similarly, the obtained data had a high degree of conformance with data obtained via internal testing, confirming the validity of historically accepted values and the verification practices employed by Ceramco for both production materials.

Additionally, the material performance was stable and predictable, with good correlation between the tested fastener cross section and the Tensile or Torsional strength.

Results

Tensile

Tensile strength is not often requested by potential customers since ceramics are not traditionally used for their performance in this area. However, tensile strength is one of the most commonly used measures of material performance which will allow for more appropriate design and use if known, or if retrofitting an existing assembly using other materials. Further, understanding the tensile strength of ceramic fasteners will help Ceramco continue to innovate by establishing a baseline against which new or novel materials may be measured.

As noted above, the test facility recorded and reported the maximum values obtained during destructive testing.

Being sufficient for most ceramic fastener purposes, the high purity alumina (A998) fasteners were tested and analyzed first, returning an average tensile strength of ~15.4 ksi. Tensile strength in this range was anticipated based on known and established alumina ceramic material properties.

Of particular note is the tensile strength of Ceramco’s aluminum oxide ceramic fasteners when compared to other common insulating materials. The Ceramco alumina sacrifices no tensile strength while exhibiting significantly higher maximum service temperature. For example, when compared to Polyether Ether Keytone (PEEK) Ceramco’s alumina has a comparable tensile strength, but an operating temperature over three times higher (16 ksi /480°C vs. ~15.4 ksi /1650°C).

Making use of zirconia (ZrO2) in the manufacture of ceramic fasteners allows Ceramco to enhance fastener performance for more rigorous and/or unique applications. Testing reported an average tensile strength of 21.7 ksi for Ceramco’s stabilized zirconia fasteners. Comparatively, this represents a 29+% increase over the average tensile strength observed in the tested alumina fasteners.

For customers seeking to load ceramic fasteners in a purely tensile mode, or whose application may induce tensile loading during use or operation, Ceramco has delineated the below listed Proof Strengths. Customers should ensure that their application does not exceed the values listed.

High Purity Alumina |High Purity Zirconia 
SizeProof (psi)|SizeProof (psi)
#0-8018.06|#0-80- - -
M2 X 0.426.86|M2 X 0.438.17
#2-5628.15|#2-5639.97
#4-4037.01|#4-4052.34
#5-4043.7|#5-4061.69
M3 X 0.544.94|M3 X 0.563.42
#6-3246.58|#6-3265.71
#8-3259.91|#8-3284.33
M4 X 0.760.81|M4 X 0.785.55
#10-2466.86|#10-2494.04
#10-3273.3|#10-32103.02
M5 X 0.878.89|M5 X 0.8110.86
1/4-2092.6|1/4-20129.99
M6 X 1.094.76|M6 X 1.0132.99
5/16-18121.33|5/16-18- - -
M8 X 1.25129.82|M8 X 1.25- - -
3/8-16149.13|3/8-16- - -

Torsion

As mentioned above, monolithic ceramic materials are inherently brittle. This fact alone makes proper installation torque critical to the function and utility of a ceramic fastener, so it is of little surprise that torque requirements are chief among fielded fastener-related inquiries at Ceramco. Luckily, torsion is the most tangible and easily measured mechanical property an end-user is likely to encounter while working with fasteners. Established limits allow a customer or end-user to simply set a torque wrench and install the fastener without over-tightening and shearing the head off.

As with tensile testing, all torsional testing was destructive. Recorded torsional data on alumina fasteners displayed good correlation with historic data gathered internal to Ceramco, returning torsional strengths between approximately 2 and 44 in*Lbf., respective of fastener size.

As with tensile testing, the results from torsional testing showed increased performance in the zirconia fasteners. On average, zirconia exhibited 28% more torsional strength than a corresponding size alumina fastener. It should be noted that, while there is an apparent inversion point in the comparison of ZrO2 versus Al2O3 around Ø.230, this is due to a slight over-performance in the ¼”-20 tpi alumina fasteners tested.

Ceramco recommends consistent and progressive loading when applying torque to ceramic fasteners, as erratic or off-axis loading may lead to premature fastener failure. It should also be noted that perpendicularity and parallelism between joined members is crucial to mitigate application of bending moments which can have a deleterious effect on ceramic materials. Ceramco has established the below listed (maximum) service torque values for customers whose application(s) may be dependent on specific torque or preload requirements.

High Purity Alumina |High Purity Zirconia 
SizeSCV. Torque (in*Lbf)|SizeSCV. Torque (in*Lbf)
#0-800.13|#0-80- - -
M2 X 0.40.35|M2 X 0.40.5
#2-560.4|#2-560.56
#4-400.5|#4-401.15
#5-401.27|#5-401.79
M3 X 0.50.98|M3 X 0.51.93
#6-320.5|#6-322.13
#8-321.98|#8-324.24
M4 X 0.71.97|M4 X 0.75.48
#10-244.09|#10-245.76
#10-321.95|#10-327.45
M5 X 0.84|M5 X 0.88.4
1/4-2013.45|1/4-2012.31
M6 X 1.010.95|M6 X 1.015.43
5/16-1825.74|5/16-18- - -
M8 X 1.2527.04|M8 X 1.25- - -
3/8-1640.42|3/8-16- - -

Strength of ceramic fasteners

Ceramco manufactures both custom and stock fasteners out of alumina and zirconia, and although these are high performance materials, they are not going to have strength properties similar to your traditional fasteners.  The tensile strength and/or torque values for these products have not been reported inasmuch we’ve only been able to advise customers on their usage, such as, be careful not to over-tighten them.  And it was up to the customer to be able to translate the typical mechanical properties of these materials to evaluate the material limits.

This is no longer the case!  We now have torque values to report to customers (we did actually provide a small chart with torque-to-failure values before, but those data were from in-house testing, and not to be confused with an installation torque).  We also have tested the tensile strength, which previously had not been reported in any form.  Individual papers on the torque strength and tensile strength will be published in the near future based on the test results.

In general, the fasteners performed as expected and in the case with torque values, were at least in correlation with previously measured values from over 10 years ago.  The strength of the zirconia fasteners compared to the alumina fasteners was lower than expected, and has raised the question as to how fasteners made in ZTA (Zirconia Toughened Alumina) will perform.  ZTA is a composite which combines the strength and toughness of expensive zirconia with the good performance and low cost of alumina.  Testing of a ZTA fastener is underway and to be reported in a later paper.

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